1. What does it mean?
Short for 성공한 덕후, literally a 'successful fan.' It celebrates a fan getting a special connection with someone or something they admire, such as meeting a favorite artist or working in a field they have long loved.
2. The nuance
Success here is an affectionate fan-world description, not a ranking of who is the most devoted. A small personal interaction can feel just as meaningful to the speaker as a big career milestone.
3. Examples
“좋아하던 작가한테 직접 사인을 받았어. 나 성덕 됐어!”
I got an autograph in person from my favorite writer. A fan dream came true!
“어릴 때부터 보던 팀에서 일하게 되다니 완전 성덕이네.”
Getting to work for the team you've followed since childhood? That's a real fan dream come true.
Sources
National Institute of Korean Language
References support the meaning and usage. Example sentences are written for NoCapWiki.