Green’s Dictionary of Slang

carpet n.2

[rhy. sl.; carpet-bag = drag n.1 (5); or f. the earlier assumption that prison workshops took just 90 days to produce a particular type of regulation size carpet. But note No. 77 Mark of Broad Arrow (1903): ‘Your “Auto-leyne” cares little about a “drag” (three months), a sixer (a “carpet” it is generally called), or a “stretch”’. However, note Michael Quinion (WWW 6/5/00): ‘I suspect No 77 made a mistake, since “carpet” came about as the result of a bit of rhyming slang: “carpet-bag” = “drag”, implying that the two words have always meant the same thing’]

1. [20C+] (UK prison, also carpeting) a three-month sentence.

2. [1940s+] £3, £30.

In phrases

double-carpet (n.)

[1970s+] (UK prison) six months’ imprisonment.