1. What does it mean?
덕밍아웃 blends 덕후 with coming out. It means revealing a fandom interest that was previously hidden, either deliberately or through an accidental giveaway such as a wallpaper or a familiar song lyric.
Also written or said as deongmingaut
2. The nuance
Here it refers to disclosing a hobby. It should not be taken as equating the experience or stakes with disclosing someone's sexual orientation.
3. Examples
“회의 중 배경화면 때문에 덕밍아웃했어.”
My wallpaper accidentally revealed my fandom during the meeting.
“같은 팬인 걸 알고 나도 덕밍아웃했지.”
Once I learned they were a fan too, I admitted that I was one.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.