An older, heavy-set woman. Very similar to a MILF, but without the wanting to have sex with them part. Can be any chunky woman, but usually reserved for fat blondes.
Sure Dan's wife is a butterbomb, but it's not like he has a whole lot of other options.
Person A: Hey man, I've been hooking up with this hot chick all week
Person B: Pics or it didn't happen
Person A: Nah man, she's a butterbook, wait until you meet her