1. What does it mean?
웃참 shortens 웃음 참기. It names the effort to keep from laughing, whether the setting calls for quiet or someone is deliberately trying to make you break into laughter.
Also written or said as 웃음 참기, 웃참 실패, utcham
2. The nuance
웃참 실패 means the person could not keep a straight face. In video titles it may describe a comedy challenge, but in conversation it can simply mean suppressing a laugh without any organized game.
3. Examples
“녹음 중에 배에서 소리가 나서 다들 웃참하느라 힘들었어.”
Someone's stomach growled during the recording, and we all struggled not to laugh.
“고양이가 카메라를 가리는 장면에서 웃참 실패했어.”
I lost the try-not-to-laugh challenge when the cat blocked the camera.
Sources
Careet: Korean slang and culture
References support the meaning and usage. Example sentences are written for NoCapWiki.