Tappet
Engine part and workplace slang for distracted person
Moving part of an engine that rattles when loose resulting in engine failure. Slang usage as someone who's tappets are slightly loose (not quite with it - a.k.a someone who is a few sandwiches short of a picnic) South African variation is of a tappet being a redneck in the american sense of the word
Donny is a real tappet. Harvey is so tappet