This words stems from the guy named Harold on the movie "Kids". It's where you pull down your pants and start slapping your semi-hard penis back and forth off each leg. You can also use it during sex while your cumming, slapping the girl in the face.
A reference to Harold Ramis, the actor who played Egon in Ghostbusters, the act of you (Harold) swinging your balls an inch above your friend's face while he sleeps as you chant "I ain't 'fraid of no ghost" repeatedly until he wakes.
A bonus would be the friend waking up with his first reaction to lift up his head which would ram into Harold's dangling/swinging nards which is when Harold must respond with, "he slimed me".
As Nic was sleeping, Carey pulled down his pants, squatted over his face and gave Nic a Dirty Harold
when your hanging out with your friends and you get bored so you strip to your underwear and start grinding on them, turning them on and eventually you all have a big gang bang but with spicy twist, you and your friends put taco powder on eachothers dicks and smack eachother while calling eachother juan
last night i did the dirty harold with my boigage, Russell, and julian rodierojomcde