Derived from the word "mundane", a mun is the author writing for a character in an online RPG. Can also be used as a suffix to denote a reference to a character's player rather than the character. Synonymous with certain connotations of roleplayer, author, player.
"That mun may be an idiot, but I can't fault his playing style."
"Do you think Wren-mun's coming tonight? I want to finish this RP with her."