1. What does it mean?
비공굿 shortens 비공식 굿즈. It usually refers to fan-made or independently produced items connected to a fandom, rather than goods officially released or licensed by its rights holder.
Also written or said as 비공식굿즈, bigonggut
2. The nuance
Unofficial does not automatically mean counterfeit: an original fan-made design and a fake official product are different things. Nor does the label itself establish permission to reproduce protected material.
3. Examples
“친구가 직접 디자인한 비공굿을 보여 줬어.”
My friend showed me the unofficial merch they designed themselves.
“비공굿을 공굿인 것처럼 소개하면 안 되지.”
You shouldn't present unofficial merch as official merchandise.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.