Green’s Dictionary of Slang

woodman n.

[prisoners were once forced to spend the first month of their sentence sleeping on bare boards]

(UK Und.) a prison sentence of one month.

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[UK]J. Gosling Ghost Squad 25: Thieves’ argot, spoken properly, is a foreign language which needs to be learned [...] Each prison sentence has its own word [...] ‘woodman’ for one month (from the fact that years ago in prison you slept on wooden boards for the first month).