Mechanic is an insulting slang term for a dealer who is skilled at handling cards and uses that skill to cheat. The logic is simple: a car mechanic fixes cars, and a card mechanic “fixes” card games so they go his way. In short, calling a dealer a mechanic means calling them a cheat.
That makes it a word to avoid at the table, unless you really are talking about someone who repairs cars. Dealers at reputable casinos follow strict procedures set by the house and by the gaming regulator of their jurisdiction, and surveillance staff watch their hands closely. Questioning their honesty with this word would be a serious insult. In online and live dealer games, fairness rests on certified RNGs, automatic shufflers and dealer procedures recorded on camera.