1. What does it mean?
막내온탑 pairs 막내, the youngest, with on top. Fans use it for a youngest member who is especially bold, doted on, or able to tease older members without losing their affection.
Also written or said as mangnaeontap
2. The nuance
It describes a playful group dynamic, not formal seniority or a license to be disrespectful. The same member can be both indulged and caring toward the others.
3. Examples
“형들이 다 양보해 주네, 완전 막내온탑이야.”
The older members all let them have their way: the youngest really is on top.
“게임에서만큼은 막내온탑 분위기였어.”
During the game, at least, the youngest was clearly in charge.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.