1. What does it mean?
입덕 marks the start of a fan interest: the moment or process of going from casual curiosity to actively wanting more. The 입 (入) conveys entering, while 덕 connects it with 덕후, a devoted fan.
Also written or said as 입덕하다
2. The nuance
입덕했어 is a natural way to say 'I became a fan.' It does not require joining an official fan club. The opposite, 탈덕, means leaving that fan interest behind; 덕질 is the activity of being a fan.
3. Examples
“라이브를 우연히 듣고 그 밴드에 입덕했어.”
I happened to hear the band play live, and I became a fan.
“나를 입덕하게 만든 장면이 바로 이거야.”
This is the scene that got me hooked as a fan.
Sources
The Yonsei Chunchu: fandom culture
Edaily: Korean fandom expressions
References support the meaning and usage. Example sentences are written for NoCapWiki.