Green’s Dictionary of Slang

(UK prison) a one-month sentence.

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 12: Wooden ’un: A months imprisonment.
1950		P. Tempest Lag’s Lex. 232: woodener. A man doing one month’s imprisonment. In the ‘old days’ a prisoner was given a wooden spoon with which to eat. Also, at one time, the first part of a sentence was served without a mattress. The man slept on the wooden bed-board. There is also a rhyming connection between three words connected with a month’s sentence ‘woodener’ — one who uses a wooden spoon — ‘moon’, and ‘spoon.’.