The term mav term originated from a character named 'Mav' who was very well known for accidentally saying things, sending tells or pages or speaking on a public forum something meant for someone else...like a mischannel or replying to the wrong person.
Where it comes from is not certain exactly, but there are two theories. The first holds truest, and thus is accepted as the true reason though. The second may be nothing but mere speculation, or a common application of the first.
The True Origin of Maverick (Mav) Edit
Maverick was a player on the second Vampire the Masquerade TinyMush back in 1993. The name of the Mush was the famed Elysium. TinyM**'s of many varieties allow you to set whats called an alias for easy communication - an abreviation if you will. Maverick's alias was "Mav". Whether this is the same Mav mentioned below in FuzzyLore is difficult to say with any certaintiy, but it would not surprise this Elder.
Maverick frequently had run-ins with the members of the Staff, of which I was a member. He was something of a trouble maker and chronic rule bender. Mavericks' real problem wasn't that he put colons in the wrong places. Instead, it was that Maverick would frequently send complaints to his friends - or at least - he was trying. In reality, good ole Mav was normally so stupid or stoned, he was sending the complaints to the very people he was complaining about. Since the alias is included in private communications known as pages, the behavior was quickly characterized by those now famous three letters, Mav.
No. You can not. Have. An Attack Helicoptor.
True story. I was there, for all of it.
The Legend Of MavEdit
Mav had one small problem: that of placing a colon or quotation mark before typing fairly often. The result? Whispers and pages became public says or shouts, thus informing the wrong person, or everyone around, what was meant to be told to another player through private conversation.
And so the name of Mav gained the meaning of a miscommunication. To 'MAV' was to mistell, miswhisper, shout, or other such by accident. The meaning of the verb has since changed to include making any say/whisper/page/pose typing confusion.
Example: Mav shouts "I love the moment when I have worn my underpants long enough when they just suddenly feel like they are a part of me." (Public-shout)
(yes, that was really said by Mav.)
The other possible origin (though it is rarely belived; nearly everyone agrees with Mav being the originator) is from military use. To 'MAV' is to use a 'Mis-Appoprated Verb' - i.e. a communications mixup. Most likey this was adopted after Mav was coined, and is in fact a backronym.
This 'accident' was so common that it became known as a 'mav', saying 'whoops, mav' or just simply 'mav' to this day means accidentally saying something in the wrong place.