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4.
A friend of a friend (like on facebook) that's not actually friends with you but you have to hang around with them anyway.
"Do we have have Dave here, he's a massive twat"
"Yes we do! He's your friend-in-law now"
by Felix da Pope October 03, 2011
 
1.
The best friend of your wife or girlfriend, similar to a sister-in-law.
Are your coming with us to the game?
No, I have to go over to my friend-in-law's house to put up storm windows.
by Stephen Schaffter June 09, 2005
 
2.
A good friend of a good friend. Even if you tend to hang out in the same circles often, this person would only qualify as more of an acquaintance.

Not someone you would desire to hang out with alone. Either because you only talk to the person large group settings and never really directly bonded, or because it's someone with whom you share nothing in common (other than a similar friend) and it would be awkward and uncomfortable to spend time alone together.

Most often it is a significant other's best friend.
"Nah, I don't see Megan anymore ever since Dave moved away. She was only a friend-in-law anyway."
by Jeff Jism November 05, 2007
 
3.
An individual of the oppiste sex from yourself, only your friend because they are currently dating your friend.
Meagan, my friend-in-law called me last night because she was wondering what to get Scott for Christmas.
by Jaaang August 28, 2007
 
5.
a person who is a friend of an in-law, but who hates your guts.
jorge:why don't you talk to rebecca?
micheal: she's just a Friend-in-law. I don't owe her any respect.
by cookiesandcream123 August 01, 2012