Boyfriend-In-Law 

You're (biological or unbiological) sister's boyfriend.

Meghan's unbiological sister is Tori. Tori is dating Patrick. Patrick is Meghan's boyfriend-in-law.

Boyfriend-In-Law by MeghanMurder May 17, 2010

Boyfriend-in-law 

When your wife is a whore that can't make it through your military tour without getting dick, so she ends up obtaining a boyfriend to fill in her dickspace despite her marital vows.

My boyfriend-in-law fucks my wife while I go fight for my country.

Boyfriend-in-law by Cuntrybarb787 October 25, 2019

Boyfriend-in-law 

When you are talking to somebody about your sister in law's boyfriend and not wanting to include him as part of the family.

I am tired of my Boyfriend-in-law, I wish they would break up.

Boyfriend-in-law by I-HATE-MINE October 20, 2010

Boyfriend-In-Law 

How you are introduced to people at a party by your girlfriends sister, when the parties in question are over 21 years old and very committed to each other.

Hey Sally, this is my Boyfriend-In-Law,(name), and hopefully soon he will make my sister an honest woman.

Boyfriend-In-Law by Supertcher June 15, 2009