(historical) A person whose job is cleaning cesspools or sewers, or emptying privies by night.
Near-synonym: shit-carter
1722, Daniel Defoe, A Journal of the Plague Year, London: E. Nutt (etc.), p. 54,
That the Laystalls be removed as far as may be out of the City, and common Passages, and that no Nightman or other be suffered to empty a Vault into any Garden near about the City.
1825, Robert Mudie, London and Londoners; or, A Second Judgment of “Babylon the Great”, London: H. Colburn, 2nd edition, 1836, p. 197,
Still [the rats] are an organized race, and can combine together for the purposes both of attack and defence; as the nightmen who clean the sewers and cesspools of Babylon often find. The travels of a nightman have never been published; but they would make a very curious book. As they worm their way along those dismal passages, they often find an army of rats drawn up to oppose their farther progress […]
1851, Henry Mayhew, London Labour and the London Poor, London: G. Woodfall & Son, Volume II, The Street-Folk, p. 193,
I have met with nightmen who have told me that there was “nothing particular” in the smell of the cesspools they were emptying; they “hardly perceived it.” One man said, “Why, it’s like the sort of stuff I’ve smelt in them ladies’ smelling-bottles.”