Top Definition
The relationship a boyfriend has to his live-in-girlfriend's parents.
Girlfriend: Mom, Andrew and I are moving in together

Mom: Oh, you're going to live together before marriage? Great, how I've always wanted a SIN-IN-LAW.

Girlfriend: But we love each other!

Mom: That's great, your love can help keep you warm right alongside the BURNING FIRES OF ETERNAL DAMNATION
by green duck butter November 21, 2016
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