1. What does it mean?
맏내 blends 맏이, the eldest, with 막내, the youngest. It describes the oldest person in a group acting cute, mischievous, or youthful enough to remind fans of the youngest member.
Also written or said as mannae
2. The nuance
The word is affectionate and depends on the contrast with actual age order. It does not mean the person is literally the youngest or incapable of responsibility.
3. Examples
“제일 형인데 장난은 제일 많이 쳐서 맏내야.”
He's the oldest but jokes around the most, so he's like the group's oldest-youngest.
“맏내가 분위기를 풀어 줘서 다들 웃었어.”
The playful eldest lightened the mood and made everyone laugh.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.