Someone who's constantly out getting dirty racing dirtbikes, running races, fourwheeling, etc.
Also, the
line of dirt you get on your neck from being active
outside, doing
stuff.
"Hey, where's
Danny? I thought we were going to that new oxygen
bar this
afternoon and then watching the box set of Friends?"
"Oh, he's up on the mountain running his KTM down the south slope - he's such a dirtneck."