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.
by MeghanMurder May 16, 2010
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.
by Yankee Fan in Orlando June 15, 2009
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.
by I-HATE-MINE October 18, 2010