1. What does it mean?
감다뒤 shortens 감 다 뒤졌네, using a coarse word for dying to say someone's sense is gone. It can criticize an unfunny joke, a badly judged decision, or a performance that disappoints an audience.
Also written or said as gamdadwi
2. The nuance
This is significantly sharper than 감다살. The death wording is figurative here, but still rude; do not assume a stranger will hear it as harmless banter.
3. Examples
“내가 만든 썸네일 다시 보니 감다뒤였네.”
Looking back at my own thumbnail, my judgment was way off.
“친구 농담에 감다뒤라고 했다가 말이 너무 셌다고 사과했어.”
I called my friend's joke hopelessly off and then apologized for being too harsh.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.