Green’s Dictionary of Slang

nailer n.

[lit. and fig. uses of nail v.]

1. [19C] a clincher, a ‘knockout’.

2. [mid-19C] a policeman.

3. [mid-late 19C] an extreme example, whether good or bad; applied to people, animals or objects.

4. a severe blow.

5. [late 19C–1920s] an extortionist.