backup chapstick

A side hoe

Someone who is considered a 'spare significant other' if the first one goes away

Say there are three people (A, B, C). A is dating B, but has been putting enough effort into a relationship with C that if A and B breakup, A can be in a relationship with C.

"I heard that Samantha has been leading Ben on even though she's dating Nick."
"Yeah, Ben is her backup chapstick"