Patron: You how doing something poorly is called doing it "half-ass?" What do they call it if you do a good job: full-ass or no-ass?
We actually do have quite a few slang dictionaries and word origins books, but I honestly couldn't tell if this was a serious question or not. Not that I should treat it any differently, but I'm generally paranoid and had a suspicion that someone was putting me on just to see how far I'd take this.
So instead of hitting the authoritative print resources ("authoritative," in this case, is a relative term), I started with internet searching.
I first went right to the Urban Dictionary and searched for half-ass, hoping the definitions would also list antonyms. They didn't (nor did half-assed). So, I looked up both full-ass and full-assed, and no-ass and no-assed.
Based on those entries, we concluded that the opposite of half-ass is full-ass; or, "to perform at an exceptional level of quality."
However, wanting to verify this conclusion against another source, I did an internet search for "opposite of half-assed" and found wordie.org, "a social network for people who love words." Their definition of half-assed included the line "The opposite of full-assed."
This corroboration was good enough for both me and the patron (who seemed genuinely interested the entire time), so I never even checked our print resources.
Tax dollars well spent, yes?