Green’s Dictionary of Slang

(US) a partial euph. for jackshit n.

1986		W.D. Ehrhart Passing Time (1988) 206: A bunch of kids [...] who didn’t know jack squat about real responsibility.
2002		Cheat Code Central 🌐 The manual tells you jacksquat, which means each character has to visit the Danger Room in Academy Mode to learn their powers.