1. What does it mean?
만렙 combines full with a shortened form of level. In games it means reaching the current level cap. Outside games, attaching 만렙 to a skill or trait jokingly describes someone as having mastered it.
Also written or said as 만레벨, mallep
2. The nuance
A game's level cap can change, and reaching it does not mean finishing every activity. Figurative 만렙 is praise or exaggeration, not an actual qualification.
3. Examples
“드디어 만렙을 찍어서 다음 콘텐츠를 보러 갔어.”
I finally reached the level cap and went to check out the next content.
“동생은 길 찾기 만렙이라 지도를 잘 봐.”
My younger sibling is a navigation expert and reads maps really well.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.