Named after Mav - an early TinyMUD character known for frequent and embarrassing misdirected messages - a mav is a page or pose sent to the wrong MUCK or person, or even from the wrong alt on a MUCK. A private whisper spoken out loud is an example of a mav.
In use, one often says "mav" alone after a misdirected comment.
Mavs are often humorous and revealing. It's not uncommon to be in the middle of a semi-formal discussion for example, only to have someone who's gone largely silent suddenly pose that she "slides her hand up your thigh and feels your warmth."
A folk etymology  for the word is as an acronym for "Mis-Appropriated Verb" used in World War 2-era communications. This backronym appears in no historical documents or lists of military abbreviations, and has been debunked by early TinyMUD players and Mav himself.