Green’s Dictionary of Slang

1. (US) nonsense, trivial matters.

1976

M. Braly False Starts 355: Don’t shuck an old friend. They don’t violate for that sort of nitshit.

2. (Aus.) a term of abuse, a mediocrity.

1982

P. Radley My Blue-Checker Corker 40: ‘Things are always bloody grim in England, ya nitshit’.