Green’s Dictionary of Slang

nab n.2

[nab v.1 (3)]

1. a police officer; thus the nabs, the police.

Spirit of Public Jrnls 17 178: [from British Press29 July 1813] A nab stepp’d in and show’d his writ— / The Poet ’gan to curse a bit.
[UK]‘Nocturnal Sports’ in Universal Songster II 179/2: Give the nabs the double though [...] and generally cut out escape.
[UK] ‘The Blowing In Quod’ in Swell!!! or, Slap-Up Chaunter 40: When Nab, he was down upon me, / But not to have a to do.
[US]‘Ned Buntline’ Mysteries and Miseries of N.Y. I 41: I don’t know nothin’ about no persuits, ’cept the nab’s persuits.
[UK]Western Mail 16 Oct. 3/7: The ‘ossifer’ fails to find his whereabouts for a while, thus he mitches, and in his own phrase ‘has high jinks along with the “nabs” making the dock’.
[US]‘Hal Ellson’ Duke 54: He don’t look like a nab.
[US]‘Hal Ellson’ Tomboy (1952) 142: Chickie, the nabs!
[US]N. Algren Walk on the Wild Side 244: Another walked past a nabber with a bill in his hand, and the same nab [...] let him pass.
[US]W. Brown Teen-Age Mafia 20: You’d think the nabs would keep an eye on the place.
[US](con. 1953–7) L. Yablonsky Violent Gang (1967) 12: Even those who wanted to throw me in the Hudson River [...] for being a ‘stoolie’ to the ‘nabs’.
[US]R.R. Lingeman Drugs from A to Z (1970) 188: nab An arresting officer, policeman.
[US]E.E. Landy Underground Dict. (1972).
[UK]J. Morton Lowspeak.

2. (US) an arrest, a police raid.

[Aus]Truth (Sydney) 11 Mar. 4/5: At his eleventh hour he suffered nab, / A beastly curious and persistent copman / Would not believe he was a languid shopman.
[US]D. Burley Orig. Hbk of Harlem Jive 107: They [...] ducked all nabs.
[US]N. Algren Walk on the Wild Side 75: When I put on the steam [i.e. whistle] [...] it means drop everything, it’s the nab.

3. (US) a railroad security man.

[US]N. Algren Walk on the Wild Side 78: One noon an armed nab stuck his nose in a box-car door — ‘Come on out of there one by one!’.