What does 형설 (螢雪) literally mean?

 ANSWER: fireflies (螢) [and] snow (雪)

My Korean-English dictionary defines 형설 (螢雪) as "diligent study," but the phrase literally means "fireflies (螢) [and] snow (雪)." The phrase is based on old Chinese stories about two men who were too poor to afford lamp oil to study at night, but both were so eager to study that one of them would gather "fireflies" (螢) on summer nights to study by their light, and the other would study in the winter by the moonlight reflecting off the "snow" (雪), so the phrase 형설 (螢雪) came to mean "diligent study."

From Dong-A's Prime Korean-English Dictionary