An exclamation to indicate a spelling or grammatical error.
Root of the word lies in repeated typing of "mying" instead of "mine". Over time this has come to be used to reflect any typo or grammatical error that someone makes. A term of abuse, whenever someone makes any sort of error in their email, they will expect a reply of "MYING".
"I went to the shop today and some stamps and cards" - MYING - left out "bought"
"Can somoene explain Liverpool's success?" - MYING - "someone"
"Is anyone still there" - MYING - no question mark
"What is you're problem?" - MYING - spelling