1. What does it mean?
덕후 developed through Korean adaptations of Japanese otaku. It now appears with many interests, from animation and idols to stationery or trains, to describe a dedicated enthusiast.
Also written or said as 오덕후, deokhu, deok-hu
2. The nuance
It can be a proud self-label, affectionate teasing, or an unwanted stereotype. Passion does not automatically mean expertise or obsession. Avoid assuming that somebody wants to be called a 덕후 just because they enjoy a hobby.
3. Examples
“문구 덕후라서 여행 가면 작은 문방구부터 찾아봐.”
I'm a stationery enthusiast, so when I travel I look for little stationery shops first.
“우리 오빠는 기차 덕후라서 노선도를 보는 것만으로도 신나 해.”
My older brother is a train enthusiast. Even looking at a route map gets him excited.
Sources
DukPlace: K-pop fandom glossary
References support the meaning and usage. Example sentences are written for NoCapWiki.