1. What does it mean?
레전드 borrows 'legend' and works as enthusiastic praise for a person, moment, or performance. In joking reactions it can also mark a spectacular failure or absurd situation.
Also written or said as 레게노, rejeondeu
2. The nuance
The word alone is not always positive. 레전드 실수 points to a memorable mistake, whereas 레전드 무대 usually praises an outstanding stage performance.
3. Examples
“모두가 함께 부른 마지막 곡은 레전드 무대였어.”
The final song everyone sang together was a legendary performance.
“케이크 촛불을 켜기 전에 꺼 버리다니 레전드 실수네.”
Blowing at the birthday candles before they were lit was an unforgettable blunder.
Sources
Sinjoeo Korean Slang Dictionary
References support the meaning and usage. Example sentences are written for NoCapWiki.