Interesting question, but a very obscure one! However did you encounter this term?

"Cicisbeo" is a vernacular onomatopoeic expression meant to replicate the sound of frivolous conversation (the Italian words Chiacchierare and the dialectal Ciacolare have the same root). The more "correct" job title would be "Cavalier Servente," which more or less translates to "Gentleman Servant." And the job entailed just that, a manservant who was tasked with serving and accompanying a lady.

You see, the head of a leading aristocratic family in 18th century Italy might seldom be in the same place as his spouse. A permanent presence at court, political and administrative responsibilities, participation in the monarch's retinue in visits abroad or sojourns in various state residences far from the capital, in addition to overseeing the smooth running of his own personal estates and business ventures, meant that aristocratic spouses could spend lots of time apart. And in the 18th century, a married aristocratic woman would also keep a busy schedule of public events, religious ceremonies, family functions, as well as visits to personal friends and favorite relatives. Thus while her husband was at court she might be visiting her family estate in the country, and when her husband was overseeing his own estate she might be in the city. The rigors and risks ties to travel for an unaccompanied aristocratic woman (be they in the city or in the country) in addition to the ordinary social anxieties tied to attending engagements alone (anxieties magnified at a social rung that placed a disproportionate value on social performance) made the presence of a pleasant male companion desirable.

Enter the Cavalier Servente: a man who could organize, oversee, and protect a woman as she travelled, accompany her to social events, and more generally provide genial and pleasant companionship. Commonly, the Cavalier Servente could be a member of a very minor or poor aristocratic household (in short, a well-connected man who could not afford the upkeep of his own household). Many were former military officers, while others still were educated professional servants. But whatever their background, the important thing is that they had developed the skills necessary to make good conversation within aristocratic circles, as well as understand and anticipate the needs and desires of their charges in order to help organize their affairs and help manage their social schedule.

What would the Cavalier Servente be paid? Probably a small stipend. But the major perks tied to the job would be delivered in-kind: room and board in the great aristocrat's household (as well as room and board when visiting others), in addition to access to the most exclusive events of the aristocracy, expanding the Cicisbeo's social network and possibly opening doors to a lucrative appointment in business or government.

It is difficult to assess how commonly a Cavalier Servente developed a sexual relationship with his mistress. It certainly did happen, but it's difficult to evaluate the frequency. High-profile aristocrats who competed for choice appointments within the administration of the Italian States could find their spouses' fidelity subject to rumors spread by rivals. Foreign commenters, especially the English who might not have the best grasp of the Italian Language and customs, reached broad ill-informed conclusions about the practice (French writers, on the other hand, being typically more acquainted with the language exhibited much less surprise about the practice). But most importantly, the influence of the French Revolution and the subsequent rejection of aristocratic excesses meant that from the very start of the 19th century, the public mood even amongst the aristocracy found the employ of a Cicisbeo ridiculous and decadent. In the 19th century, social as well as economic factors began blurring the line between aristocracy and upper middle class, and women were given more responsibilities. While opportunities might not yet open in business or politics, in the running of the household and family affairs spouses would have more and more contact with each other, becoming true partners and in the ideal case, confidants. With spouses spending so much more time together, there was no need for a "Gentleman Servant" to accompany a woman anymore. It's also worth pointing out that as intellectual and social discourse turned to notions of Italian Nationalism and the end of servitude to foreign powers, the Cicisbeo also became an trope tied to a social degeneration and an allegory for longstanding political servitude.

But if nineteenth century writers scorned their parents' practice, in the eighteenth century when it actually happened the selection of the Cicisbeo was an ordinary process tied to the setting up of a young aristocratic household. It was something akin to arranging for a house and the hiring of servants. Naturally, a vaguely aristocratic and well-educated background was preferred (however, a background that was too aristocratic could be embarrassing) and a genial and trustworthy demeanor was necessary (as the expectation, at least, was that the relationship would be platonic). The profession was thus not only a refuge for second sons of minor households, but also a chance for a great house to extend their network of patronage and favors; when vying for an important appointment at court, for example, the head of a great household could count on support from his friends and extended family, but maybe also a peer whose destitute nephew he hired as Cicisbeo for his daughter (indeed, the examination of the importance of social and extended family ties in the pre-unification political system in Italian States is its own topic of discussion).

The role of a cicisbeo was primarily to serve as an escort to a married noblewoman, meaning that he would attend her, provide conversation and amusement, and accompany her to dinners and other public events in the absence of her husband. This was not an adulterous relationship because it was publicly recognized and even expected and carried out with the full knowledge of the husband, who may have even had a hand in selecting him. The cicisbeo sometimes even occupied apartments in the same house as her husband and had the right to command his servants. Nobles elsewhere--especially in France--sometimes had similar arrangements, but the cicisbeo was unique in that it was a publicly recognized and sanctioned role.

This was not a paid position, as they were drawn from the nobility themselves, but the husband usually financed their outings and activities. Nor was it always an exclusive arrangement, as some of them were married themselves or served as an escort to multiple women.

To what degree the relationship was sexual, making a "love triangle" out of the marriage is ambiguous. Critics of the practice in other European countries treated it as such and took it as evidence of depravity of the aristocracy in general and the sexual immorality of Italians in particular (a common stereotype in the early modern period)--but this is not necessarily a reliable source for the nature of the custom. Roberto Bizzocchio has a book on the cicisbeo that tries to disentangle whether or not the cicisbeo were just polite companions or if the relationship was also sexual based on primary source accounts--and it seems that the answer is yes, although he argues that we should understand this threesome as a normal and expected model of marriage in the eighteenth century, consistent with standards of noble behavior, rather than a sign of sexual freedom or choice. In other words, it was taken for granted that the practice opened the way for love and sex between the two, but it was not treated as the central role of the cicisbeo, which was as an escort first of all.

I would also hazard a guess that the custom made perfect sociological sense considering the restrictive marriage customs of the Italian aristocracy. Noblewomen often married very young to much older husbands, often in the interest of making a political match. Furthermore, women were often expected to only appear in public with a male escort, a service which the cicisbeo could provide. As married couples may have been completely indifferent to one another, it makes sense why they might prefer to socialize with others when out in "polite" society.