Green’s Dictionary of Slang

1. a clincher, a ‘knockout’.

1821–6

‘Bill Truck’ Man o’ War’s Man (1843) 262: Hang me, Joe, but that’s a complete nailer to ye, me lad.

2. a robber.

1825

Pierce Egan’s Life in London 2 Jan. 389/1: Not more than ten persons left the metropolis, and the screws and nailers upon the road; who had previously calculated upon doing summut, were most miserably disappointed.

3. a policeman.

1825		‘Margate Steam Packet’ in Universal Songster I 14/1: Oyster dealers - fish-fags - sailors / Gentry - tradesmen - porters - nailers.
1847		Era (London) 15 Aug. 3/3: The nailers [...] they are up to everything.
1863		T. Taylor Ticket-Of-Leave Man Act I: dalton: The Crushers are getting to know too much; then there’s the nailer’s been after me. moss: What, Hawkshaw, the ’cutest detective in the force.
1935		A.J. Pollock Und. Speaks n.p.: Nailer, a uniformed police officer.
1960		C. Cooper Jr Scene (1996) 308: Nailers: police.
1966		‘Railroads have “Slanguage”’ in Newark (OH) Advocate 21 May 3/3–4: nailer – a railroad detective.

4. a severe blow, a knock-out punch.

1827		Standard (London) 20 Oct. n.p.: That’s right, Harry — go it — serve him out [...] tip him the nailer — show him the mill.
1853		Westmorland Gaz. 7 May 5/5: One of the ‘boys’ gave Billy such a nailer with his shillalah on the back side of his head that Billy was nailed to his mother earth.
1896		Regiment 18 Apr. 42/2: Thin rushin’ in, he hot [hit] him one uncompromisin’ whack, / That stretched him out immadiately full length upon his back, / An’ (the poor baste) before he cud say “ Nailer,” he whips a grate big knife.

5. an extreme example, whether good or bad; applied to people, animals or objects.

1849		Bell’s Life in Sydney 1 Sept. 3/2: One Joseph Housley, a nailer (a dead nailer), of Sussex street, and a very old sinner beside.
1886–96		Marshall ‘Pomes’ from the Pink ’Un 88: At guzzling the whole lot were nailers [F&H].
1890		Sporting Times 3 May 3/1: My! She is a nailer, and such diamonds.
1897		[perf. Lottie Collins] A Leader of Society 🎵 Well, I'm Missis Talbot Taylor, a regular, right down nailer.
1898		H.B. Vogel Maori Maid 150: She’s a perfect nailer! She’s one of the prettiest girls I’ve ever seen.

6. an extortionist.

1888

Illus. London News Summer No.26 3: The Stomach of the Bar, collective and individual, is revolted and scandalised at the idea of its members doing anything for nothing. Yes, put in Eustace, I have always understood that they were regular nailers [F&H].