A jam jar is a car. It is a piece of Cockney rhyming slang, built on the fact that “jar” rhymes with “car”, and it has long been particularly associated with London speech, including the language of criminals talking about a vehicle used in a robbery. Someone might talk about “getting the jam jar out” to mean simply driving somewhere, but the term has also had a more specific underworld use, describing the car used to get away from a crime.
What’s the origin of the phrase ‘jam jar’?
The rhyming-slang use of “jam jar” for “car” is dated by Tony Thorne’s Dictionary of Contemporary Slang to the 1920s, and he notes that it was still current in working-class London speech at the time of his entry. Green’s Dictionary of Slang gives a slightly more specific early history, recording that the term was first applied to a tram car before it broadened out to cover a motor car generally. That sequence fits the way rhyming slang usually develops, starting as a piece of very local, spoken usage attached to one particular thing before spreading to a wider sense once the original object, in this case the tram, had become less familiar.
The phrase’s association with crime is well attested rather than incidental. Thorne’s dictionary preserves an example of the term taken directly from a criminal informant in London, describing a getaway car: “He had to blag a jam jar for the getaway.” Here “blag” means to steal or obtain by deception, so the sentence describes arranging a car to escape after a job. This kind of quotation shows the phrase doing real work in the speech of people for whom naming a car in code, even a mild and jokey one, had practical value.
The Free Dictionary’s idioms entry gives the same standard explanation found elsewhere, that the phrase comes from rhyming slang pairing “jam jar” with “car”, and notes that it is heard primarily in the UK. Beyond the general history of rhyming slang as a form, there is no further mystery to the coinage: it belongs to the large stock of Cockney rhymes built on ordinary household objects, sitting alongside other everyday items pressed into service purely because they happened to rhyme with the word needed.