Definition of luckbox
A "luckbox" is a player who runs well, who is lucky, to whom everything smiles. The expression can be used for several reasons. If you hit a four of a kind on the river to beat your opponent's set of aces, you're a luckbox. If, on the turn, you had a 4% chance of hitting your four of a kind on the river and you hit it, that's luckbox. If you're a beginner player who wins the WSOP main event by winning almost every hand, you're a luckbox.
Don't confuse luckbox and skills
The expression luckbox often has a pejorative connotation. It means "he won the tournament because he's a luckbox". It's possible to be lucky once or twice, but when you win consistently, it's not because you're a luckbox, but because you're doing something right. Many players, out of frustration, won't recognize their opponents' quality of play and rather than admit to themselves that they made a mistake and that their opponent played well, they'll pretend that their opponents are luckboxes. In the long run, everyone will have their luckbox moments!