Green’s Dictionary of Slang

(US) to hold back, to doing things by halves.

2010		A. Steinberg Running the Books 32: I’m not saying the community of my youth is loud and pushy. Only that they’re in earnest. They don’t half-ass things.
2013		C.J. Ross ‘Three Days Ahead’ in ThugLit Nov.-Dec. [ebook] ‘Were they half-assing it so they could move on to someone more worthy?’.