There's another meaning given by Chino in an old interview (loudside.com), here's what he said about the name 'Mx':
Chino: "The song was first called "Max", because when we wrote the song, the riff was similar to something that Max (Cavalera, Soulfly singer) would make. When we went in the studio, Stef took out the "a"... and it's now called "MX"..."
So there you go, I guess this is the most credible meaning for the weird name 'Mx'. (thanks to Autopsy17)
"MX" is also an abbreviation for MotoCross, one of Chino's hobbies. (thanks to Drew Birch)