Green’s Dictionary of Slang

badly, unsatisfactorily.

1924		E. Hemingway letter 6 Dec. in Baker Sel. Letters (1981) 138: The only feeling I have about the boy is feeling sorry I acted so shitty to him.
1940		E. Hemingway letter 4/11 Feb. in Baker Sel. Letters (1981) 501: If I wrote as sloppily and shitily as that freckled prick I could write five thousand words a day.
1967–8		Baker et al. CUSS 195: Shitty, do Do poorly on something [...] on an exam.