1. What does it mean?
넘 replaces 너무 in conversational writing. It modifies a quality or feeling, as in being very happy or finding something too difficult; 넘넘 adds playful repetition.
Also written or said as neom
2. The nuance
It is not the numeral 'number' and does not always mean excessive in a bad way. Use 너무 in standard writing, and let the sentence determine whether English needs 'so' or 'too.'
3. Examples
“네가 보내 준 엽서 그림 넘 귀엽다.”
The drawing on the postcard you sent is so cute.
“상자가 넘 무거워서 둘이 들어야겠어.”
The box is too heavy; we'll need two people to carry it.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.