got no jams

What Rap Monster says to Jimin on their plane to L.A.

You are no fun

The origin of this phrase "you got no jams" comes from korean word "재미 (jaemi)" which means "fun". Jaemi and jams sound familiar, so Rap Monster put jams instead of Jaemi in the sentence making it funnier.

P.S : In korean, "you got no jams" is "너 재미없어 (neo jaemi obseo)"

A very terrible insult. If you have no jams, on a scale of Jimin to Namjoon, you are Jimin.

It’s an inside BTS joke made by RM and Jimin.

That means that someone is not funny. As you know you can also use it in your Real life cause why not actually.

U have become a jamless person in life.

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 highest level of insult possible.
Literally meaning you have no fun. You are a lifeless peice o' shit.