Green’s Dictionary of Slang

(US) a keen young police officer eager to make arrests.

1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 441: savage, n. A rookie cop who is wild to make a pinch.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 184/2: Savage. Any newly appointed police officer overeager to make initial arrest.