Ergo...

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... 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.

Source: From Two and a Half Men.

Explanation: Alan is dating Kandi, a 22-year-old who isn't that bright. She comes walking in the door and yells that she's there for a booty call, not knowing that Alan's son, Jake, is in the house. Jake asks Alan what a booty call is, and Alan immediately says that Kandi has come over to do laundry. Jake asks why she doesn't have a laundry basket with her. After a long pause, the following conversation ensues:

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 (Alan's brother): 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?