Sherlock Holmes, n. — Green’s Dictionary of Slang

Sherlock Holmes n.

[proper name Sherlock Holmes, the private consulting detective invented by Sir Arthur Conan Doyle (1859–1930)]

1. (US black) the police.

[1957		G. Kersh Fowlers End (2001) 304: ‘Isn’t that blood I see on the floor?’ asked Inspector Dench. Sam Yudenow started to say ‘Sherlock Holmes—’].
1980		E. Folb Runnin’ Down Some Lines 67: Characteristic of the police-related lexicon is an ironic, if sometimes grim, humor that is embodied in names like [...] Uncle Nab, nail ’em and jail ’em, Sherlock Holmes.

2. (N.Z. prison) an excessively inquisitive prison officer.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 164/2: Sherlock Holmes n. a watchful, over inquisitive prison officer.