Why "boo" and where the heck did it come from?

As far as terms of endearment go, I have a real problem understanding why people would choose to call their significant other, “Boo”. I did have an SO some years ago who almost always called me that and it drove me up the wall.

My understanding of the word, “Boo”, is either something that is used in Scooby Doo cartoons, or the longer “Booooo” which is used to denote widespread disappointment towards, typically, a team in sports or a ref making a bad call.

I’m not great on terms of endearment, so can any of you spill the beans on this one? Is it short for something? Does it mean something? What? I don’t get it.