The origin of Booty Call dates back to the times of Cowboys and Indians.
Alan : You see Jake, in the old west, cowboys could be out on the dusty range for months at a time, and they get mighty dirty. So they'd mosey into town with nothing but the clothes on their backs, and they'd need to wash them. So what they'd do is, they'd go down to the creek, and strip down until they were wearing nothing but their boots.
Charlie: Why'd they keep their boots on?
Alan: Rattlesnakes. Anyways, in order to warn people that were swimming that a naked cowboy was on his way, he would yell, or, if you will, call, "Booty! Booty!" Ergo, the Booty Call.
Kandi: Wow Alan, you really make history fun.
Alan: Well, thank you.
Kandi: So when did it start meaning casual sex?