1. What does it mean?
Short for 썩은 미소, 썩소 describes a smile that looks mocking, cynical, or forced. The contrast between a smiling mouth and an unfriendly feeling is the point.
Also written or said as 썩은 미소
2. The nuance
It is an interpretation of an expression, not proof of what somebody intended. It can be a playful description of a fictional character or a friend's deliberately exaggerated pose. Calling a stranger's ordinary smile 썩소 is likely to sound insulting.
3. Examples
“악당이 마지막 장면에서 썩소를 지었어.”
The villain gave a sneering smile in the final scene.
“일부러 썩소를 지으며 사진을 찍었는데 너무 웃겨.”
I posed with a deliberately sarcastic smile, and the photo is hilarious.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.