My first thought after reading the question was the same as what Andreas said. It makes sense to me, especially when you think of the term "Booty call" - when you meet up with something, possibly an ex-lover, just for sex. There a movie with that name a few years ago. By the way, I'm an American female, in my mid-twenties, so maybe there's a difference of opinion between cultures or generations? I'm not into trendy or "slang" speech, so if that's what I think of, that meaning or idea has to be fairly widespread.
It seems to me the idea of "booty" as in treasure or loot, while correct, is a little outdated. Ask the average person from younger generations (like thirties and younger) and they'll probably think of either sex or a person's butt, and would probably think of treasure or loot only if the context made it obvious. And I've never heard of "booty" referring in any way to a childhood friend or anything similar.
"--> when booty means:
butt... then "booty pal" could mean a childhood friend... rt from the time that you were born/ were toddlers.... "
I'm sorry, not trying to be rude here, but I don't get the connection between "butt" and childhood friend, toddlers, etc. at all.
I'm not trying to say the sexual meaning is the only one, but I might argue it's more common among certain people or age groups. If R. would be kind enough to provide more context, we could decide what it means in his example.