Any person who shows no emotion, no facial expression, and makes absolutley no eye contact during intercourse. They basically just lay there completley dead weight, "not having sex", acting like a total nun about it.
"Dude, how was last with Andy?"
"I cannot believe I had to fake blowing my load, she was a total nun fuck dude."
1.N; the act of having sex with a really hot chick who is not at all into you. it should be exciting, but it turns out feeling like you're fucking your own sister, or, worse, a nun who got drunk and broke her vow of chastity, not because you're so hot, but because their life sucks THAT bad.
He finally got to fuck that supermodel, Sabrina, b ut he said it sucked because the whole time she was tweeting her friends that she was in jamaica, what a Nunfuck.