Green’s Dictionary of Slang

(US prison) the bread and water diet given as a punishment.

1934		J.L. Kuethe ‘Prison Parlance’ in AS IX:1 26: dry. A diet of bread and water once daily for the infraction of a prison rule.
1950		Goldin et al. DAUL 63/1: Dry, n. (Middle Atlantic States, except N. Y.) Bread-and-water diet in punishment cells.