look up any word, like bukkake:
 
4.
term used for a "good" female friend who is closely related (e.g. sister, best friend, roommate) to your girlfriend
Dude, you're dating Lindsay but I saw you face-sucking with Britney.

It's cool -- Brit's my girlfriend-in-law -- that gives me special privileges!

If you can convince Lilo of that, you are my new god!
by cyberpope67,BC,Canada August 23, 2010
 
1.
When one of your friends starts dating a girl and she all the sudden becomes totally cool with you/conversates with you.
On the scope of available females she just became about as useful to you as your sister. Unless you want to fuck your sister. But if your the kind of person who wants to fuck your sister you probably would fuck your friends girl too.

Anyways this shouldn't become confused with homie hoppers. A girl can only be one of these at a time. Most homie hoppers start as a girlfriend in law but can transform into a homie hopper. But they can NEVER be both at the same time...JH
Girl: Hey how are you?
Me: Bitch I don't cerrr'. I dont wanna be your buddy just cause you fuckin my friend. Quit trying to waste ma damn time ho.
Girl: okay see ya around tee hee (starts skipping away) tralala lala lala

Bitch faced Mitch: Yo, you hittin that?
Me: Nah my homeboy is. She's just my girlfriend in law.
Bitch faced Mitch: word, well that sucks
by NegrasGrande January 09, 2012
 
2.
The girl you practically have a relationship with in an emotional level; but without the ability to kiss, or perform any sexual acts, due to ill circumstances or she's taken.
Friend: Sup dude where are you off to?

Me: I'm off downtown to see my girlfriend in law. I promised I'd take her out for a beer and then maybe some cuddling on the beach at night.

Friend: Haha good times but, you guys are just kidding yourselves. Get together and be done with it.

Me: Dude you know she has a boyfriend.
by 1stOfJuly July 01, 2013
 
3.
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.
by Mighty Dekunut August 05, 2014