Green’s Dictionary of Slang

an unpleasant, underhand person.

1958		F. Norman Bang To Rights 15: Without being a shnide at that.
1962		F. Norman Guntz 36: I [..] have divided the geezers who run the theeatre into [...] shnides, thorough shnides and double thorough shnides.
1979		F. Norman Too Many Crooks Spoil the Caper 2: Some shnide fitted us up good an’ proper.