Green’s Dictionary of Slang

wooden (spoon) n.

[rhy. sl.; wooden spoon = moon n. (2a)]

(UK prison) a one-month sentence.

1934		P. Allingham Cheapjack 245: The fair-people referred to three month’s imprisonment as ‘a carpet’; a month’s as ‘a wooden.’.
1992		R. Puxley Cockney Rabbit.