Green’s Dictionary of Slang

1. a bully who specializes in finding victims at night.

1616		Webster Duchess of Malfi II i: If you hear the common people curse you, be sure you are taken for one of the prime night-caps.
1624		R. Davenport City-Night-Cap (1661) IV 41: So a City-Night-Cap; for whilst he doth rome And frights abroad, his wife commits at home.

2. a thief who prefers to work at night.

1915

M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 209: night-cap [...] a thief.

3. a prostitute.

1915

M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 209: night-cap a [...] harlot.

4. (US Und.) a night watchman.

1949

Monteleone Criminal Sl. (rev. edn).