look up any word, like goatse:
 
1.
A girl who you will become your wife if the two of you are single at a pre-determined age.
Robin: Let's make a pact, if we both turn 40 and we're both single..
Ted: Robin Scherbatsky, will you be my Back-Up Wife?
Robin: A girl always dreams of hearing those words. Yes, yes, a million times, yes!
by drewdaspot March 03, 2012
9 -1