Green’s Dictionary of Slang

(Irish) a (stolen) car.

2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 78: One o’clock in the morning, roysh, they head back to the jammer.
2005		P. Howard Curious Incident of the Dog in the Nightdress 36: If that jammer’s still got a full set of alloys [...] when I come back, there’s ten sheets in it for you.