A not-boyfriend. A boy who'll spend all his free time with a girl, shag her loads, say lovely things to her, even go on holiday with her - but then vociferously deny that anything is going on between them, even though it appears to the outside world as if they've having a relationship.
N.B. Girls can also do this to boys, but it seems to be more common the other way round.
"So what's going on with Jim and Becca then? Are they going out?"
"Well - when they're out in public he'll pretend he's not with her, so he can scope out other girls, but they spend all their time together, and it's obvious they really like each other. He's totally noyfriending her."