Well, this peculiar acronym expression stands for two different things. In the World of Warcraft game world, "mt" has traditionally meant "main tank", a type of combat role that a player takes on during group combat. But in the game world of Final Fantasy, SWTOR, and other games, "mt" also stands for "mistell" or "mistyped", a way of saying "sorry, I did not type that last message correctly". Mistells occur when the user hit some wrong keys while typing a chat message, and they quickly want to tell people to disregard their last note. The mt expression, like many other Internet expressions, is part of online conversation culture.
Reminder: 90% of the time, these expressions are typed in all lowercase letters. On selective occasions, you are welcome to use them in all capitals to express enthusiasm. Just remember not to type entire sentences in all caps, lest you be considered rude.
Related: 322 other common internet expressions explained.
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