"재미없어" (jae mi eop seo) means "It's not funny/fun". A more casual slang variation of this is "노잼" (no jam). The first phrase is a combination of "재미" (fun) and "없어" (does not exist), which directly translates the phrase to "fun does not exist". "재미", the first word, is abbreviated to "잼", and we attach it to the English word "no", resulting in this shorter and more fun way to say that something is not.
"맛있어" is a phrase that should be familiar to many Korean learners - it means "It's delicious". But some dishes are so good that it deserves an upgraded way to describe it - "존맛탱" is the phrase you are looking for! You can also simply say "존맛", which is an abbreviation of "존나 맛있어". The first of the two words is a slightly vulgar slang word to say "very"). There are a few assumptions as to why people started adding the last character "탱", but for now we'll leave it as a word to emphasize.
Koreans also further abbreviate the phrase by using the phrase's English initials - JMT.
As mentioned above, this phrase includes an abbreviation of a vulgar word, so use the phrase with that in mind (i.e. people that you are comfortable and close with). That said, it's a guaranteed way to make your Korean friend laugh!
"킹받아" is a combination of a word and a phrase: 1) 킹, which is the Korean pronunciation of the English word "King" and is used as an emphasis (like "very") in this context, and 2) 열받아 (yeol bad a), which means "It's frustrating/annoying".
This phrase is relatively new - it's known to be commonly used by Korean Gen Z's. Do note that this has a fun note to it, so use it only when you're slightly annoyed, or annoyed in a joking manner.
"웃기다" (ut gi da) means "it's funny", and "아프다" (a peu da) means "it's painful". Put these two phrases together and you get "웃프다" (ut peu da), which describes an emotion where you're sad in a way that makes you laugh too.
We may sum up this honorable mention's definition with a meme:
and a Korean meme that represents the "웃프다" phrase: