Scythe is a relatively modern tool in comparison to the sickle that hails from the neolithic times, but it still seems to appear in Western Europe in the mid-Latene period (around 4th century BC). They were most likely well known around 1st century AD in Britain, as specimens found at Newstead and Abington are dated to Flavian era and display characteristics of the Gallic scythe, i.e. they have long, thin blades and have mounting tangs protruding at roughly right angle from the heel of the blade proper (it is possible that they were meant to use it like a sickle, but the shape of the blade suggests mounting on a longer handle). In his "Natural History', Pliny the Elder mentions that there are two popular version of a scythe, a smaller one used by Romans (or rather people living on Italian peninsula) and the larger variant commonly used by Gauls. Earliest medieval depictions of scythes follow kind, as seen here: (https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Wandalbert_von_Pr%C3%BCm_-_Juli.jpg/800px-Wandalbert_von_Pr%C3%BCm_-_Juli.jpg). This is one of the miniatures adorning a martyrologium written by Wandelbert of Prüm and presented to the emperor Lothar I, what means that it must have been done somewhere between 848 (when the manuscript was finished) and 855 (year of Lothar's death). Given that the reaper with a scythe accompanies text describing the month of for July strongly suggests that this tool was already widely used along the sickle (portrayed on the subsequent illustration) and could have already been a shorthand for the agricultural work, although other historians tend to place the popularization of that farming tool in Central Western Europe to 12th or even early 13th century. But once the tool has become popular, it started to be used as a makeshift weapon when necessary.

Now, let me make a small distinction between the terms 'scythe' and 'war scythe' that, in my opinion, is necessary, as iconographic evidence strongly suggests that apart from regular scythe blades, people using such an improvised weapons often utilized similar tools possessing curved blades, such as chaff cutters (prior to introduction of the crank-operated cutters in mid-19th century, they had a form of a curved blade mounted on a pivot located on one end) or even drawknives. Please note that the scythes in their standard configuration (blade positioned at more or less right angle relative to the snaith) were almost never used due to sheer impracticality of such weapon that would be fragile and unwieldy, especially in a formation. This is why I will use the term 'war scythe' when referring to the weapon. In addition, there is at least one known specimen of a scythe-based weapon that is not a polearm. I'm referring to the sword or rather sabre allegedly belonging to Thomas Müntzer, a leader in German Peasant Wars and made of a scythe blade fitted so that it is located towards the wielder with a hilt that somewhat resembles one used in 16th century Hungarian sabres.

The earliest mentions of the war scythes along other makeshift weapons used in large numbers are usually associated with Hussite Wars in Bohemia (1419-1436) and German Peasant War (1524-1526), with both being conflicts that saw mass mobilization of the peasants who were neither trained not equipped as any regular fighting force and had to rely on the modified tools. The war scythes were also quite commonplace in popular uprisings in the 18th and 19th centuries. A good example can be the Polish-Lithuanian Commonwealth and its former territories under foreign jurisdiction after the partitions. In popular imagination, they are most commonly associated with Kościuszko's Uprising that was broke out in 1794 as a reaction to the second of the aforementioned partitions or, more precisely, with the peasant militia formed out of roughly two thousand inhabitants of the Krakow region. These soldiers were primarily armed with the modified scythes as they lacked the military training required to used the rifles that were, nevertheless, not available in a number required to issue each militiaman with one. The number of militias quickly rose and in the Battle of Szczekociny on 6th June 1794 Polish side fielded more than 6000 of militiamen armed with war scythes and pikes. Similar militias were also quite popular during the November Uprising and resulting Polish-Russian war in 1830-1831 as well as the Uprising in Greater Poland in 1848 and January Uprising of 1863-1864. War scythes were also used by peasant militia in e.g. Popular Uprising in Transylvania in 1784-1785, Vendee Wars during the French Revolution, Uprisings in Tirol in the year 1703, 1805 and 1809 and the Jutland invasion by the army of Schleswig-Holstein in 1848. Of note is also the battle of Sedgemoor in 1685, as it was not a popular uprising but rather the rebellion of the pretender to the throne that happened to utilize substantial number of peasants armed with modified farm implements although scythes were also commonly accompanied by modified sickles, pitchforks or pikes. The last recorded 'official' usage of war scythes as weapons occurred in 1939, when the company of militiamen formed by the Polish Socialist Party in Gdynia were to use scythes in attacks against Germans to capture military-grade weapons.

Interestingly, the usage of war scythes and other improvised weapons was to some extent codified by Chrystian Aigner, Polish professor of the military engineer academy who laid out the basic tactics in his book Krótka nauka o kosach i pikach [Short treatise about scythes and pikes]. When used in a tight formation, such apparently primitive weapons could have been quite effective, provided the morale of the users were high. As the late 18th century muskets were still hardly impressive when it comes to accuracy, a charge with pikes and war scythes could have been as devastating as the charge of the soldiers armed with bayonets, provided the attacking unit did not falter due to attrition under fire. It can be argued that if the assaulting unit was not disrupted by enemy fire, it could have even been at an advantage due of the much longer reach of their weapons. According to the conscription act of the Order Commission of the Krakow Voivodship, each recruit should have been armed with a firearm or an 11-foot pike or a modified scythe mounted on the handle 8 feet long, suggesting that special purpose snaiths were used to match the length of pikes, as the snaiths of the regular scythes usually matched the height of the user, meaning they were more or less 6 feet long. In the battle of Racławice, such a charge resulted in the capture of the entire battery of 12 Russian cannons, heavily contributing to the Polish victory.

Thus, the war scythes were most definitely used on the European battlefields by the common folk belonging to more or less organized troops, especially since late 17th century sometimes even being used as a weapon of choice but usually only when other weapons were not available.

Obviously, the Scythe is primarily an agricultural instrument and not a weapon. However, we have fairly good evidence that the scythe was considered an improvised weapon that might occasionally be encountered. There are even a few plates in the 16th century fencing codex of Paulus Hector Mair showing scythe techniques. 16th century German fencing manuals are known for depicting a variety of weapons, some of which are a little bit silly, so it's tough to know how serious to take such depictions. Still, we can at least be sure that the idea of fighting with scythes is not (just) some modern media trope but an idea that also existed in the pre-modern age.