While I’m sure women are sick and tired of leery men complimenting them on their asses, this prank plays on their prior expectations. And works brilliantly as a result.
The set-up is that a guy casually walking down the street says something about an ass in earshot of some girls. They immediately assume he’s talking to them, reacting negatively to the unwanted attention.
He then sidesteps past them and heads for the actual object of his admiration: an ass, as in the animal.
Most of the girls take it all in good nature, laughing once they realize their mistake. But one or two of them are guilty of overreacting just ever so slightly.