Where does "buttload" come from?

This may sound like a weird question, but it feels like there's so many answers. Is it from "boatload"? Is it from the fact that donkeys can also be referred to by a word also meaning "butt" and they carry a lot of stuff? There's also things that say it's an exact measurements, but also things saying that it isn't exact. I'm very lost. Does anyone know, or is it super complicated? Thank you in advance to anyone who can help.