U got no jams

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

The most insulting insult in the kpop world pretty much...ever.
The word originates from Rap Monster (BTS). In korean the word fun sound a lot like the english word jams. That is why Rap Mon told Jimin (BTS) that he has no jams. Meaning that he is "very no fun" (J-Hope said this about Jimin).
Saying U GOT NO JAMS means you are telling someone that they are not fun.

A phrase used towards one people to tell them that they are not fun in ones eyeballs but maybe not anothers but they are your baby and you wouldnt give them up for the world