1. What does it mean?
A combination of English 'no' and 잼, from 재미 ('fun'). It gives a blunt negative reaction to a joke, a show, a game, or another experience that the speaker does not find entertaining.
2. The nuance
This is a subjective reaction, not an objective review. Calling a person 노잼 can hurt more than describing a film that way. A gentler alternative is 내 취향은 아니야: 'it's not really my thing.'
3. Examples
“이 영화는 나한테 좀 노잼이었어.”
I found this movie a little boring.
“그 농담은 노잼이야. 다른 얘기 하자.”
That joke isn't funny. Let's talk about something else.
Sources
National Institute of Korean Language
References support the meaning and usage. Example sentences are written for NoCapWiki.