OK so first a reminder. A Croupier is the actual and official name of the dealer in a casino. Here is where it comes from.
Medival knights generally traveled fairly light, usually only one servant who rode behind him on the croupe. In French croupe is the horse’s rear. So those that rode on the horse’s rear were called croupier.
When a group of knights traveled together and stopped to rest for the night, gambling would be the way to pass their time. The croupier would draw lots among them to determine who would be the dealer for the night, and thus get the tips.
The name stuck. Knowing this now means that the next time you go to the casino you simply must restrain yourself from calling the dealer a horse’s ass. Unless they deserve it of course.