What does this idiom mean?

GUEST   Sun May 04, 2008 1:33 am GMT
Beat a person at his own game
What does this idiom mean?
By the way, would anyone recommend a good website which explains the meanings of idioms? Thanks!
Guest   Sun May 04, 2008 3:48 am GMT
It means outdoing an expert or champ in their own field of excellence.

Example: Suppose you've never played golf before, and enter the US open. You manage to win despite the odds, beating all the pro golfers at their own game.