1. The forums at www.musicianforums.com, as referred to by members of the site. This is as distinct from mx (small case), who is the site's admin. see also MXican
2. A song by Deftones, from their Around the Fur album.
Dude, this one time, on MX... (the beginning of one of many MX stories doomed to not being understood by anyone outside the forums)