I didn't notice there was already a thread discussing this! Linking to that!
Also, mods, DELETE if you can!)
(Original gist of my post):
What might the shirt mean:
1) Mutant (obvious)
2) Mirage (The publishing studio of TMNT; maybe some kind of Arabian-esque Mystic Knight)
3) Time Mage (Classic game, Turtles in Time) <-- My money's on this.