In many sea shanties I've listened to, like this one, for example, I've heard references to flash girls. When I google this term, I either get tattoos or pornography. So what does it really mean in the context of this shanty?
As referred to in this particular song, a “flash girl” is a prostitute. It is clear from the lyrics of the song. The term “flash” in the 1800s was a British cant term covering members of the criminal fraternity. The song also makes several references to the girl’s “flash man” or pimp.
FLASH-MAN: a favourite or fancy-man; but this term is generally applied to those dissolute characters upon the town, who subsist upon the liberality of unfortunate women; and who, in return, are generally at hand during their nocturnal perambulations, to protect them should any brawl occur, or should they be detected in robbing those whom they have picked up.
Reference: A Vocabulary of the Flash Language, J.H.Vaux, pub 1819