Used as an insult. If you say someone is ‘spare parts’ then you’re calling them
useless, or they’re so
poor at what they are doing that they might as well be doing
nothing.
In a co-op video game, if there’s someone in your party that ends up doing jack shit for the team, then you’d say they are “spare parts.”
Player 1: Dude, player 3 got no absolutely no kills the entire
round.
Player 2: I know, right? He
just sat on his ass the
whole game!
Player 1: Yup—he’s spare parts.