Why do I get a 5 minute mute for saying this in chat?

[P u r e]

mmk

A sound made by combining a mumble and the last half of "OK". Represents a distracted affirmation
"Could you do this for me?"
"Mmk..."

Please explain why typing that in game gives you a 5 minute mute. I get muted more for "mmk" than I care to count.

Note:
I'm not lieing, please don't go testing this to see if you get muted too. I guarantee that you will, just take my word for it.