1. What does it mean?
뇌피셜 blends 뇌, brain, with 오피셜, the Korean form of 'official.' It jokingly labels an idea as official only inside the speaker's head rather than established by a source.
Also written or said as noepisyeol
2. The nuance
People can mark their own speculation with this word, or use it to challenge somebody else's unsupported certainty. Calling an explanation 뇌피셜 does not itself prove the explanation false; it questions the evidence behind it.
3. Examples
“이 장면은 다음 편 복선 같은데, 아직은 내 뇌피셜이야.”
I think this scene foreshadows the next episode, but that's just my theory for now.
“출시 날짜는 뇌피셜 말고 공식 공지로 확인하자.”
Let's check the release date in an official announcement, not rely on guesses.
Sources
Nate News: Korean usage in the press
References support the meaning and usage. Example sentences are written for NoCapWiki.