Green’s Dictionary of Slang

(US Und.) a cry of warning, alerting others to the approach of authority.

1933		Ersine Und. and Prison Sl.
1946		J.E. Dadswell Hey, Sucker 85: Chiggers, the cops!
1950		Goldin et al. DAUL 43/1: Chicky or Chiggers. Beware! Look out! Police!