1. What does it mean?
금공강 combines 금요일, Friday, with 공강, a class-free period. Students use it when their timetable has no Friday classes, often enjoying the possibility of an earlier start to the weekend.
Also written or said as geumgonggang
2. The nuance
It does not mean a university-wide holiday or permission to skip class. A person can have 금공강 and still need to study, work, or attend a meeting.
3. Examples
“이번 학기는 금공강이라 목요일 저녁에 집에 가.”
I have no Friday classes this semester, so I go home Thursday evening.
“금공강이지만 팀플 회의는 잡혀 있어.”
I have no classes Friday, but there's still a group-project meeting.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.