A screw mate is a roommate who you happen to have casual sex with every now and then. It can even be a progressive thing where you start as occupying an apartment together to occupying each other’s beds and bodies from time to time.
Friend: “You wouldn’t believe what happened last night with Brad and I...”
Me: “Did you guys get into an argument about the dishes being left out?”
Friend: “No bro, we became screw mates, even though he’s straight.”
Me: “What is a screw mate?”
Friend: “You know, it’s where you’re living together as friends and happen to screw every now and then... mates who screw for fun.”
Me: “Sounds like the plot to a porn...”