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1.
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.
by anonymous7787 March 19, 2012
 
2.
Two people who having been getting together for a while but won't admit they are in a relationship
Robin is Barney's common law girlfriend on How I Met Your Mother
by Bobun 5ever August 05, 2012