1. What does it mean?
열공 shortens 열심히 공부하다. It can describe focused studying or serve as a brief message encouraging someone who is about to study.
Also written or said as 열공하다, 열심히 공부하다
2. The nuance
It suits a friend's chat or a study-log caption more than an academic evaluation. As encouragement it is often light and supportive, but repeating it to someone already stressed can sound like pressure. It says nothing about grades or study effectiveness.
3. Examples
“도서관에서 두 시간 열공하고 산책하러 나왔어.”
I studied hard at the library for two hours, then came out for a walk.
“시험 준비 중이구나. 열공하고 나중에 연락해!”
You're preparing for an exam. Study well, and message me later!
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.