Green’s Dictionary of Slang

1. (also jam) initially a tram car, a motor car .

1930		(con. 1910s–20s) D. Mackenzie Hell’s Kitchen 119: Jam jar ... a car, motor car, tram-car.
1941		G. Kersh They Die with Their Boots Clean 27: His phraseology is debased. He uses slang. To Barker [...] a car is a Jam, or Jam-Jar; talk is Rabbit, or Rabbit-an’-Pork; beer is Pig’s Ear . . . and so on, up and down the language.
1953		B. Behan Scarperer (1966) 85: I’d know that jam-jar anywhere. She has an engine like a jet plane.
1962		R. Cook Crust on its Uppers 22: For the morries [...] it’s gold kettles, the jam-jar and a kosher pad. [Ibid.] 26: We were into my jam and going like the hammers.
1963		L.F. Freed Crime in S. Afr. 104: We ‘scaled jam-jars’ (stole cars).
1971		J. Jones Rhy. Cockney Sl.
1984		‘Derek Raymond’ He Died with His Eyes Open 53: You look as if you’d had some practice with a jamjar, ha, ha. [Ibid.] 61: Where you get this jam? [...] It’s nice. You nick it?
1989		in G. Tremlett Little Legs 195: jam-jar car.
2001		Indep. on Sun. Reality 21 Jan. 9: New lad hero made it okay to like footie and books, but can’t use the jam jar.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 95/1: jam jar n. a car: ‘Put the billy lids [pi.e. kids, children] in the jam jar.

2. a farthing [= far].

1952		M.F. Caulfield Black City 122: A penny a ride or two jam jars.
1952		C. Harris Death of a Barrow Boy 62: He watched her sorting coppers with a doubtful grin. [...] ‘There, you’ll ’ave to take two farthings.’ ‘Two jam jars?’.