Someone who works at a direct care facility and helps to take care of people with developmental disabilities--especially ones with genetic cognitive disabilities. Essentially a baby sitter for retarded people.
Friend: Hey, let's go watch the game tonight!
'Tard Sitter: Sorry, bro. I can't go out to the game tonight because I picked up an extra 'tard sitting shift tonight.
A portmanteau of "mayo" and "incel". It's basically a synonym to "whitecel", or a caucasian incel.
Girl at the club: "Look at that pasty-looking dude who has no game. He must be a mayocel."