Green’s Dictionary of Slang

anything, or anyone, unpleasant or distasteful.

1936		M. Levin Old Bunch (1946) 29: Old kakers like me have to crawl around and give pills to diabetics.
1977		D. Powis Signs of Crime 190: Kaker Yiddish for excrement and thus anything unpleasant, particularly unsavoury words or exaggerated stories.