Definition of luckbox
A "luckbox" (literally, lucky box) or a player who is called a luckbox is a player who run good, who is lucky, who everything smiles. The term can be used for several reasons. If you hit a square on the river to beat your opponent's three of a kind aces, you may be called a luckbox. On the turn, you had a 4% chance of hitting your square on the river and you hit it, it's luckbox. If you're a beginner player who wins the main WSOP event by winning almost every hand, you'll be called a luckbox.
Don't confuse luckbox with skill
Expression luckbox often has a pejorative connotation. One will hear "he won the tournament because he is luckbox". It's possible to be lucky once or twice, but when you consistently win, it's not because you're a luckbox, but because you're doing something good. Many players, out of frustration, will not want to acknowledge their opponents' skill and rather than admit that they made a mistake and that their opponent played well, they will pretend that their opponents are a luckbox. In the long run, everyone will have their moments when they are a luckbox!