Green’s Dictionary of Slang

(US) a (written) statement of congratulation, e.g. I’ll write you an attaboy.

1939		W. Winchell On Broadway 11 Dec. [synd. col.] Maurice Evans carted his full-length ‘Hamlet’ back to town [...] and set the reviewers to echoing their attaboys of last season.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 28: Cheers, attaboys, holy fucks.
1990		J. Wambaugh Golden Orange (1991) 187: I’m gonna have our boss write you guys an attaboy.
1997		Simon & Burns Corner (1998) 488: It was R.C. who took in the attaboys from Tank and Mike and Tony.
2006		D. Winslow Winter of Frankie Machine (2007) 131: Maybe that was all the man had ever wanted: an attaboy, a pat on the back.
2008		Simon & Burns ‘Transitions’ Wire ser. 5 ep. 4 [TV script] ‘Hey, Scott. Good work’ [...] ‘There’s an attaboy for you’.
2015		S. King Finders Keepers (2016) 15: ‘I would like a little support here.’ ‘Maybe even an attaboy’.
2021		J. Ellroy Widespread Panic 185: ‘Your’re not stupid Robbie. That’s the only attaboy you’re ever likely to get from me, so enjoy it’.