Green’s Dictionary of Slang

1. (UK Und.) a jury.

1833

H. Smith Gale Middleton 1 160: No nose among us; and if we had I know how to gammon the twelve.

2. (US black) a year, i.e. twelve months.

1944

D. Burley N.Y. Amsterdam News 15 Jan. 10A: The cats and the bats [...] played like mad in the backbeat 12.

3. (US black gang) the police.

2016

F. Stuart ‘Dispatches from the Rap Wars’ in chicagomag.com 🌐 twelve The police, sometimes referred to as ‘the jakes’.

4. see twelver n. (1)