Green’s Dictionary of Slang

choirboy n.

1. (US Und.) a novice thief.

[US]Ersine Und. and Prison Sl.
[US]Monteleone Criminal Sl. (rev. edn).

2. (US) an innocently honest person; a naive and foolish person.

[US](con. 1948) G. Mandel Flee the Angry Strangers 426: You’re nervous, don’t kid me. Joe, you’re an old choirboy. [Ibid.] 439: [She] laughed at Joe himself, the worried choirboy waiting in the rain for her.
[US]P. Rabe Benny Muscles In (2004) 247: How would you know, choir boy?
[US]J. Sayles Union Dues (1978) 165: He’s a fuckin choirboy, Mitch, he don’t want any part of it.
[US]L. Roberts Full Cleveland 124: A guy fixes your car good, do you really give a damn whether he’s a choirboy? [HDAS].

3. (S.Afr.) an accomplice, a young man who plays a supporting role in a crime.

[SA]L.F. Freed Crime in S. Afr. 107: A ‘ringer’, ‘stooge’ or ‘choir boy’ is an accomplice.

4. (US gay) a novice male streetwalker.

[US]B. Rodgers Queens’ Vernacular 111: While starting on the beat [game, turf, walk] the inexperienced, up-and-coming choirboys, cowboys [...] wait to be approached.

5. (US) a novice police officer.

[US]J. Wambaugh [bk title] Choirboys.
[UK]J. Morton Lowspeak 38: Choirboys – rookie, i.e. newly recruited policemen.

6. (US black) a derog. term for a black person seen as embracing white values.

[US]N. McCall Makes Me Wanna Holler (1995) 281: I was a choirboy, one of those trained, assimilated Negroes white folks love so much.

7. (S.Afr. gay) a homosexual male; thus in the choir adj., homosexual.

[SA]K. Cage Gayle.