Green’s Dictionary of Slang

jammy n.1

also jammie
[abbr. jam jar n. + dimin. sfx -y]

(S.Afr.) a motorcar.

[SA]C. Hope Ducktails in Gray Theatre Two (1981) 52: Ou maat, jus’ pass me the keys for your jammie.
East Province Herald 16 Apr. n.p.: [advert] Rent a jammy. cars from R10 per day [DSAE].