1. What does it mean?
현웃 shortens 현실 웃음 and says that something actually made you laugh away from the screen. 현웃 터졌다 is a common way to describe cracking up at a clip, caption, or unexpectedly funny reply.
Also written or said as hyeonut
2. The nuance
It can mean a chuckle as well as loud laughter; it is not a volume measurement. Writing ㅋㅋ beside it is perfectly compatible with saying you genuinely laughed.
3. Examples
“자막이 너무 엉뚱해서 버스에서 현웃 터졌어.”
The caption was so unexpected that I actually laughed on the bus.
“댓글 하나 보고 현웃 나서 다시 읽었어.”
One comment made me laugh for real, so I read it again.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.