Green’s Dictionary of Slang

(US) a boring, stolid person.

1960		(con. 1940s) D. MacCuish Do Not Go Gentle (1962) 136: That broad smiled at me and this shlunk jumped in the way.
1976		H. Selby Jr Demon (1979) 92: If you think I’m just going to be some kind schlunk while that ass-kissing sonofabitch gets somewhere.