Mechanic is derogatory slang used by the players to refer to a dealer who is proficient in handling cards. However, this expression is all but nice. Namely, the term was coined by the players and is used to describe dealers who can “fix” any game in their favour. Car mechanics fix cars, while card mechanics fix card games. Simply put, a mechanic is a dealer who cheats.
So unless you’re talking about actual car mechanics in the area with someone at your table, you should be wary of using the word, especially in reputable casinos. Dealers working in prominent casinos adhere to strict rules issued by the house, as well as the gaming authorities in charge of the jurisdiction the casino is in.
Needless to say, questioning their fairness and calling one of them a mechanic would be insulting, so it’s best to avoid using this term.