What is St. Louis Backpack?
1.
When a male subject inserts his ballbag/nutsack into a female's anus and proceeds to jump on her back and latch on like a backpack.
Hey Tommy Knuckles, check out that broad over there, she has a fat ass. I think you should give her a St. Louis backpack.
Yo Vinny, my balls are killing me. Last night I gave my cumara a St. Louis backpack and she loved it.