They are documented as far back as ancient Rome, as brief holidays from daily labors. They eventually fell into ignominy during the Victorian era, decried as “schools of vice” and public debauchery. Merchants brought their wares to hawk, and entertainments ranging from musicians to bear dancing could be had. One could find – quite literally – anything at a fair. Little wonder they became ideal events to behave less than properly, or that a vulgar term came into use as its descriptor.
A fair. Also, a meeting of gamblers for the purpose of play. Frequently, any public place of amusement would be called a gaff to indicate to the rabble that coves were ripe for fleecing.
The drop coves maced the joskins at the gaff; i.e., the ring-droppers cheated the countryman at the fair.
It’s somewhat amusing and concerning that modern slang defines gaff as the word for house or flat. If you are ever invited over to someone’s gaff, perhaps caution is needed.