1. What does it mean?
무야호 is a joyful shout associated with a scene from MBC's variety show Infinite Challenge. Online, it works much like an exuberant 'woo-hoo' when someone celebrates a small win or simply feels delighted.
Also written or said as muyaho
2. The nuance
It is a sound-based catchphrase rather than an abbreviation you need to expand. Repeating or stretching it can imitate the cheer; knowledge of the old clip adds context but is not required to read its happy tone.
3. Examples
“밀린 일을 끝내고 혼자 무야호를 외쳤어.”
I finished my backlog and let out a little 'woo-hoo' to myself.
“친구가 합격 소식을 보내서 무야호라고 답했어.”
My friend sent news of passing, so I replied with an excited 'woo-hoo!'
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.