The parents of your live-in boyfriend/girlfriend.
I am having my sinlaws over for dinner...I hope I make a good impression since I have been shacking up with their son for a few weeks now and they are not down with the fact we are living in sin.
by PunTiff August 01, 2008
The parent of a partner in a romantic and cohabitating but unmarried relationship.
Sadly, John's sin-laws dropped by the house for Jane's birthday.
by Jim Dempsey August 28, 2004