Green’s Dictionary of Slang

(US prison/Und.) a lookout man.

1924		G. Henderson Keys to Crookdom 64: The ‘jigger man’ shot the policeman. [Ibid.] 409: Jiggerman. Lookout.
1931		Irwin Amer. Tramp and Und. Sl. 109: Jigger Man. –The lookout for a gang of thieves.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 110/2: Jigger. 1. One who stands watch; the act of standing watch; an exclamation of warning [...] Jigger-man. A lookout; one who stands watch to warn accomplices of impending danger.
1962		Ragen & Finston World’s Toughest Prison 805: jigger man – A lookout for other thieves.
1969		B. Jackson Thief’s Primer 53: A jigger [...] means a lookout, someone you put on the point that notifies you in case the police are approaching.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 208: jigger, n. – a lookout to warn others by relay.
1992		P. Earley Hot House 254: Bucklew needed a ‘jigger,’ someone to watch for the guard.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Jigger: (1) Warning of staff approaching. (2) One who watches while illegal act takes place.