When you and your buddies stop somewhere to pee, "Dirty Dick" is the act of picking up a handful of dirt and tossing it in your buddy's pants before he can zip up.
It is customary to shout, "Dirty Dick!" as you do it... then run like hell.
A few times in my life, I have Dirty Dicked a friend. Hurry up and finish peeing yourself, then grab a handful of dirt. Pick your slowest peeing friend and toss the dirt into his pants as he is finishing up, while shouting, "Dirty Dick!"