Q From John Abbeydale: Why does the word carpet mean three in English slang — as in a prison sentence of either three years or three months or, even, a three-pound bet?
A To find the origins of this we have to delve into criminals’ and prison slang. The first recorded use of carpet for a prison sentence comes from the book The Mark of the Broad Arrow by “No 77”, published in 1903. He suggests that the word was then current prison slang for a six-months’ sentence, with the usual term for a three-month one being drag. However, he was either wrong or carpet took over the latter’s meaning. 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.
Drag dates from near the end of the eighteenth century; J H Vaux said in his Flash Dictionary in 1812 that drag was the name given to robbing carts or wagons “of trunks, bale-goods, or any other property” (probably because they were heavy and had to be dragged about), and that done for a drag meant that a thief had been convicted of such a crime, for which the penalty was three months in jail.
Some people have said that a three-month sentence was called a carpet because it took that long to make one in the prison workshop, but the rhyming slang joke on an existing usage makes more sense. (It doesn’t ever seem to have meant so long a sentence as three years.)
The word was later extended to other instances of the number three. These seem originally to have been Australian and include a sum of three pounds, or odds of three to one, or car dealers’ slang for a sum of three hundred pounds.