Green’s Dictionary of Slang

(US) to talk, esp. to the authorities.

1881		Stark Co. Democrat (Canton, OH) 29 Sept. 4/3: Charley [...] was so badly chawed up by honest voters that he has not peeped about running for office since.
1895		E. Townsend Chimmie Fadden Explains 57: De Duchess she went, and never peeped bout, havin no game t’ sneak on.
1905		Spokane Press (WA) 12 Aug. 2/1: Russell Sage is 89 and he hasn’t peeped about a wasted life yet.
1911		G. Bronson-Howard Enemy to Society 289: If you ever ‘peep’ about one of us I’ll cut your heart out — un’stand?
1914		Day Book (Chicago) 23 July 2/1: Major Funkhouser, who is in right with the papers [...] has not peeped about his own stool pigeon’s knowledge.
1931		P. Stevenson Gospel According to St Luke’s 154: He hasn’t peeped.
1939		Howsley Argot: Dict. of Und. Sl.
1953		‘Curt Cannon’ ‘Now Die In It’ in I Like ’Em Tough (1958) 46: My wife’s been after her [...] but she won’t peep.
1965		R.L. Pike Mute Witness (1997) 10: One whisper that he was going to peep and his bodyguards would be the first to cut him down.