A boy, who a girl considers to be boyfriend material of showing interest but is otherwise unwilling to commit or admit feelings for said girl. Both may hang out and do boyfriend/girlfriend things, but not go steady.
Zach was Amanda's unboyfriend. They both always did things with eachother and Zach felt comfortible enough around Amanda to share his feelings with her. Just not his feelings for her.
The guy a woman hangs out with and does boyfriend-girlfriend relationship things with, but she's not really going out with him nor does she want to sleep with him.
While being able to attract men, the inability to keep them as a boyfriend.
Even though Tara likes Sam she couldn't help but antagonize him and treat him in such a way that makes their relationship impossible. With this behavior, she is definitely "unboyfriendable".