Green’s Dictionary of Slang

1. (US) in gambling, two.

1913		T.A. Dorgan in Zwilling TAD Lex. 102: Poker dice — well that’s my dish. Three ducks. That don’t beat sixes however.
1968		F. Wallace Poker 214: Duck — Deuce [HDAS].

2. (US prison) a two-year prison sentence.

1971–2

C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 203: duck, n. – a two-year sentence.