When two people have been hooking up for an extended period of time but neither will acknowledge the relationship. They like to say they are just friends but spend lots of time just the two of them and sleep together.
Dude 1: I'm busy tonight. Leah and I are going to the movies.
Dude 2: Hanging out with Leah again, huh? How is that girlfriend of yours?
Dude 1: Dude, she is totally NOT my girlfriend. We're just friends.
Dude 2: Bro, you can't bang a girl for 6 months and tell me you're just friends. She is totally your common law girlfriend.