1. What does it mean?
Borrowed from 'Muggle' in the Harry Potter stories, 머글 is used in Korean fandom talk for a person who is not deeply involved in a particular fan interest.
Also written or said as muggle, non-fan
2. The nuance
The contrast is about familiarity with the fandom, not a person's worth or intelligence. Someone can be a 머글 in one fan community and highly knowledgeable in another. Explain unfamiliar terms instead of using the label to shut someone out.
3. Examples
“나는 그 그룹에 대해서는 머글이라 멤버 이름부터 배우는 중이야.”
I'm not part of that group's fandom, so I'm starting by learning the members' names.
“머글 친구도 이해할 수 있게 공연 예매 방법을 설명했어.”
I explained how to book concert tickets so a friend outside the fandom could follow.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.