1. What does it mean?
월요병 literally means Monday sickness. It is an everyday expression for feeling sluggish, reluctant, or low in energy as work or school resumes after a weekend.
Also written or said as wollyobyeong
2. The nuance
Despite 병 meaning illness, casual use is not a medical diagnosis. It describes the familiar mood around Monday, not every possible cause of fatigue.
3. Examples
“일요일 저녁부터 월요병이 오는 기분이야.”
I feel the Monday blues starting as early as Sunday evening.
“월요병이라 그런지 아침 준비가 느렸어.”
Maybe it was the Monday blues, but I got ready slowly this morning.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.