The boy that you have sex with and do all the other relationship type things with. You are probably invited to all of his family functions yet you are introduced as just a friend. You may even live together or have children. He refuses to commit and maintains the attitude that having a "Relationship" will ruin your friendship. He may even date other women and you are expected to think nothing of it. "Un" Stemming from "Alice in wonderland's" Un-birthday. Where it's not your birthday but you celebrate like it is.
Un-boyfriend: Hey Stephanie you wanna come back to my place?
Stephanie: Aren't you living with Amber?
Un-boyfriend: Well yeah but we just share a bed we're not in a "relationship".