1. similar to being "head over heels", but in a more sexual nature.
2. to think or speak of your significant other all the time, with hints at sexual inferences; mainly when done by women.
3. to let physical and/or sexual desires for a person overtake rational thought; to think with the booty instead of the brain.
Stacy: "I really love my boyfriend; he's so cute and funny and strong, he loves me and listens to me-"
Rebecca: "You just butt over brains for this guy aren't you?"