pontoon, n. — Green’s Dictionary of Slang

pontoon n.

[card use, the game of pontoon or ‘21’]

(UK prison/Und.) a 21-month sentence.

1958		F. Norman Bang To Rights 177: This geezer was doing a pontoon.
1977		D. Powis Signs of Crime 197: Pontoon Twenty-one months imprisonment.
1989		in G. Tremlett Little Legs 196: pontoon 21 months imprisonment.