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."