The act of relieving one's self (urinating) outside. Typically the term is used by truck drivers and travelers who have driven many miles (without a truck stop or rest area) and- needing to piss, they pull over to a side road to "squirt the dirt".
Damn, Frank bought that big-ass 200 ounce bladder buster at the truck stop and the next thing you know, we have to pull over to squirt the dirt.
Having a slut or pornstar on her stomach while pulling her arms backwards. At this point she sprays dung out her dirtstar like a rooster tail from a muddy dirtbike