@zumi cowabunga could be related to surfing. Surfing could be connected to the physical range job being a blitzer like from Final Fantasy X where they have and islander aesthetic to it. Weapon of choice being the bliztzball.
Kinda wrong, as we were told the two new jobs are physical melee and caster. So the t-shirt could be either of the two or even both. However I'm leaning more towards it hinting to the physical melee