1. What does it mean?
In internet slang, 주작 questions whether a story, reaction, or result was manufactured. 주작하다 means to fake or manipulate it, and 주작 아니야? asks whether the apparent reality is staged.
Also written or said as jujak
2. The nuance
The word also has unrelated dictionary meanings. In this slang use it is an allegation, not evidence; an unlikely story may still be true.
3. Examples
“영상의 앞뒤가 안 맞아서 주작인지 확인하고 싶었어.”
The video didn't add up, so I wanted to check whether it was staged.
“신기한 사진이라고 근거 없이 주작이라 하진 말자.”
Let's not call a surprising photo fake without evidence.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.