An abbreviation of the welsh word "jaman", this is an exclamation used when a person says something with a degree of certainty, which is immediately contradicted by the words or actions of either the same or some other person, or some other event.
Friend 1: I did not sleep with that ugly girl!
Ugly Girl: I had a great time last night, call me!
Friend 2: Jam for you!
Flatmate 1: I know there are no tickets left for that comedian next march!
Flatmate 2: Well I just bought 2 tickets for the the late show online, so jam for you!