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