1 definition by Mighty Dekunut
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.
Girlfriend in law: Your boyfriend? he's my boyfriend too! You're my girlfriend in law.
by Mighty Dekunut August 5, 2014