1. What does it mean?
문송 shortens 문송합니다, itself blending 문과라서 죄송합니다. It jokes about difficulties attributed to a humanities or liberal-arts background, especially in employment or a technical conversation.
Also written or said as 문송합니다, munsong
2. The nuance
It reflects a stereotype rather than a real deficit. It can be self-mocking, but using it against someone else's major is dismissive; no academic field requires an apology.
3. Examples
“숫자 얘기하다가 문송이라는 농담이 나왔어.”
During the discussion of numbers, someone made a 'sorry, humanities major' joke.
“문송이라고 할 필요 없어, 전공이 다른 것뿐이야.”
You don't need to apologize for your major; your field is simply different.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.