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
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
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
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.