Green’s Dictionary of Slang

a nightwatchman.

1904		Marvel 12 Nov. 4: Foxy stuffed ’em in ’is blessed pocket when a nightie (night-watchman) sprung the alarm.
1951		A. Buckeridge Jennings Follows a Clue (1967) 80: Old Nightie, armed with a mop, was polishing the linoleum.