Poker slang for a card
cheat
"He fixed that deck good"
"Yup, he was a real mechanic"
A good example of this would be worm from Rounders. Worm just couldn't help himself cheating. It is because of the large amount of card mechanics that casinos insist on having one of their own dealers deal card games, it also helps the justify the rake, which is often large and quite profitable for the casino.