XO

A person (usually a female) who is used as a filler of time for someone in between relationships. Said person is usually kind-hearted and doesn't know they're an XO at the time. They're often seen as not good enough for a real relationship and used time and time again.

Jake: "Yo man I saw you hanging out with Samantha. Whats the deal with her?"
Timmy: "Don't worry, she'd just my XO until I get Julia to notice me."