Like a fuck buddy
except this person isn't your buddy
in any way shape or form. You have nothing but contempt, loathing and disgust for each other as people. Yet you call them or receive a text from them every time you're lonely or they're horny and you get it on. There will be no talking, except to trade insults. You will have no need to discuss "keeping this a secret" as you're both too ashamed to reveal this to anyone.
Ex's, long term partners and co workers can all become Fuck Nemeses.
I felt sad and lonely so I called Clive to come over and we had sex, after he called me a fat bitch. Oh, I need a boyfriend, not a fuck nemesis."
"Can't be bothered to try and pull or be nice to anyone. Time to call my fuck nemesis"