1. What does it mean?
머선129 represents a stylized dialect-like version of 무슨 일이고, asking what is going on. Reading 1, 2, and 9 as 일, 이, and 구 creates the sound play at the end.
Also written or said as meoseon129
2. The nuance
It is a surprised meme reaction, not a standard number or a model for all regional Korean speech. Its dramatic tone can be amused, bewildered, or disbelieving depending on the event.
3. Examples
“자고 일어났더니 알림이 백 개야. 머선129?”
I woke up to a hundred notifications. What on earth happened?
“갑자기 화면이 뒤집혀서 머선129라고 중얼거렸어.”
The screen suddenly flipped, and I muttered, 'What is going on?'
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.