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"머하눈 거야.. Can anybody tell me what is this sentence mean. Thks

Basically this comes from the sentence
"뭐 하는 거야?" Mwo ha nun go ya
Which is an informal way to say "what are you doing?"
its usually in a rude way though and its means like, "hey what the heck are you doing?"
sometimes, i dont know why koreans like to spell mwo like mo... its can be more cute or more mainly. depends

Let's break the sentences down first.

머하눈 거야(mo ha nun geo ya) - 머(meo) is supposed to be 뭐(mwo). 눈(nun) is supposed to be 는(neun)

It's a very casual way of saying, "What are you doing?".