1. What does it mean?
일코 shortens 일반인 코스프레, roughly acting like a non-fan. In fandom talk it means not showing how deeply you follow an artist or hobby, especially at work or around people who do not share the interest.
Also written or said as ilko
2. The nuance
The playful wording does not mean fans are abnormal. Some people simply prefer privacy or separating hobbies from professional life.
3. Examples
“회사에서는 일코해서 굿즈를 책상에 안 둬.”
I keep my fandom private at work, so I don't put merch on my desk.
“친구가 응원봉을 알아봐서 일코에 실패했어.”
My friend recognized the lightstick, so my hidden fan identity was exposed.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.