derives from a korean phrase: "jaemi eobseo" which means no fun or not interesting.
the "jaemi" itself means fun, and "eobseo" can be translated to (there is) no/not.
so basically what you do with it is add "no" in
front of the phrase,
scratch of the "eobseo" and simplify the pronounciation for "jaemi" into jams
done~
i'm not gonna explain how it's popularized, the result on top of this
one can do that just well