Top Definition
1. that other someone that a person in a relationship is diddling on the side; the dirty mistress, if you will.
2. synonym for the "free-pass fuck," that hot person or celebrity you would want to do and not have consequences while in a relationship.
Yeah, David is my boyfriend, but Harry is my significant nother.

Oh, yeah, Kevin knows that if Jake Gyllenhaal comes to town, he better not come back to the room, cause Jake is my significant nother.
by Siena K February 20, 2008
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