The odd occurence that when you are going out with someone you inherit their best friend as one of your own good friends. Often occurs more that a girl will become very close to her boyfriends best mate than the other way round.

"You and Johnny are such good friends How do you even know him?"
"Oh Hes Toms best mate so hes my Best friend-in-law"

Best friend-in-law by WooBlackbetty December 9, 2008

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Best friend-in-law

When your best friend gets married to the person of their dreams and you’re also friends with that person. Kinda like usual in-laws but likable.

At the wedding Lewton stood up to give his best friend Fischer and his new best friend-in-law Kyla a congratulatory toast.

Best friend-in-law by KingLouie Daturd August 27, 2018

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Bestfriend in law

Your bestfriend's husband/wife or girlfriend/boyfriend. So if your bestfriend is cheating then you have 2. Also if your bestfriend has a celebrity crush and says they are married that's also your bestfriend in law.

"Hi bestfriend in law" Said Beyonce

Bestfriend in law by YNW Melly's Bestfriend in law January 12, 2020

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