Girlfriend in law

When you are dating a guy who already has a girlfriend, or when you are dating a guy who is cheating on you with another girl, the other girlfriend is your girlfriend in law. Much like in-laws, you don't need to get along with the other girlfriend in order for her to be your girlfriend in law.

You: What are you doing fooling around with my boyfriend?
Girlfriend in law: Your boyfriend? he's my boyfriend too! You're my girlfriend in law.

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