1. What does it mean?
존맛탱 intensifies 존맛, itself a shortening of 존나 맛있다. The added 탱 gives the expression a punchy, playful sound. JMT is a familiar Latin-letter abbreviation of the same phrase.
Also written or said as JMT, 존맛, jonmattaeng
2. The nuance
The source phrase contains profanity even when the shortened form feels light. Use 맛있다 or 정말 맛있다 in polite or professional settings instead of assuming it is neutral.
3. Examples
“첫입 먹고 친구한테 존맛탱이라고 보냈어.”
After the first bite, I texted my friend that it was ridiculously tasty.
“친구끼리는 존맛탱이라 해도 공식 후기에는 맛있다고 썼어.”
I might say 'jonmattaeng' to friends, but in the formal review I wrote 'delicious.'
Sources
Sinjoeo Korean Slang Dictionary
References support the meaning and usage. Example sentences are written for NoCapWiki.