1. What does it mean?
덕계못 abbreviates 덕후는 계를 못 탄다. Fans use it when a devoted follower misses a special opportunity while someone less invested gets lucky, such as accidentally meeting the artist. It is a rueful joke about fan luck.
Also written or said as deokgyemot
2. The nuance
This is not a factual rule about fandom or a reason to resent other people. It works best as self-directed, exaggerated disappointment.
3. Examples
“친구는 우연히 봤는데 나만 못 봤네, 덕계못.”
My friend ran into them by chance and I missed them: unlucky fan life.
“당첨 소식만 남의 일이네, 오늘도 덕계못.”
Everyone else's names seem to get drawn; another unlucky day for this fan.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.