Green’s Dictionary of Slang

set-up n.²

[one ‘sets’ or sits it out]

(US Und.) a one-day prison sentence.

1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 189/1: Set-up, n. [...] 4. (Jails and workhouses) A one-day sentence.