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1.
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?"
by Joppaberry Cocktail May 02, 2011