'Cheese' is a term used by gamers, especially wargamers, to refer to something that they think is unfairly powerful compared to other things in the game, either because of a bad rule, or because someone has deliberatly streched the rules to arive at some unfair combination. People who are consistently cheesy are probably Munchkins.

The origin of the term is unclear, it is probably because such tactics are thought to 'stink'.