Butt nut is a term founded by Christopher Columbus on the day of January 5th, 1493. Popularized by the 2017 genre of memes including Doge, Ugandan Knuckles, etc. Napoleon brought the term to life in 1808 when he sent a letter to his wife during the battle of Waterloo just before his great defeat. The letter wrote "My sweet butt nut, if this may be my last nut, I'm glad we did in the butt.. - Your butt nut, Bonerfart.
One who is nuts for butts, that is, an anal sex enthusiast. Can refer to straight or gay people, though it is likely to be taken as offensive when used to describe a gay person.
Remember that girl I started hooking up with at the party? Turns out she's a total butt-nut. Fortunately, she's really anal about personal hygiene.