Green’s Dictionary of Slang

(UK Und.) a three-month sentence.

1938		F.D. Sharpe Sharpe of the Flying Squad 330: carpet (or Carpet Bag, or Drag) : Three months’ imprisonment.
1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1989		J. Morton Lowspeak.