1. What does it mean?
완내스 shortens 완전 내 스타일. It reacts enthusiastically when a song, outfit, room, or other choice lines up with the speaker's own taste.
Also written or said as wannaeseu
2. The nuance
This is about preference, not universal quality. It does not have to refer to an attractive person, and is lighter than declaring something your lifelong favorite.
3. Examples
“이 가방은 주머니도 많고 색도 완내스야.”
This bag has lots of pockets, and the color is totally my style.
“느린 기타 소리에 낮은 목소리까지, 완내스다.”
Slow guitar and a low voice: exactly my kind of music.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.