A linguistic irritant LXXVIII

When did referring to a vocation or activity as "my jam" start?  (i.e. "Painting is my jam.")  Why not refer to a hobby as a hobby, a job as a job, and a passion as a passion?  I hear the phrase, "my jam," and I think of strawberry jam on toast or a long guitar solo.  As my blogs illustrate, I enjoy creative writing.  I've never referred to it as my jam, though, and don't plan to.