and here you go:
1. Produced in a incompetent or desultory manner, or of something lacking in quality or substance; of low rank; incompetent.
* 1969, The End of the Road by John Barth (page 155)
the only abortion she could get even in the city would be a half-ass job by some half-ass doctor who could mess her up for the rest of her life
1. (slang) To perform a task incompletely or carelessly. Usually considered derogatory or insulting.
* 2002, Guardians of the Eagle by John D. Messer (page 102)
if you're working for me and I send you to do a job and you half-ass it, that reflects on me. In civilian life, we'd just fire you. Here I can't get rid of you.