When a male is performing intercourse on a female (missionary) and defecates into his on hand; and then while continuing to thrust at a steady pace he throws the fecal matter onto the female’s stomach with the intention for it to splatter/explode much like a wet snowball. The male may choose whether or not he would like to scream dirty snowball in celebration of his great accomplishment.
once the fecal matter smacks the female in the stomach, you have completed the "Dirty Snowball"
When a man or woman takes a ball-shaped bowl movement on their partner's chest then proceeds to ejaculate onto the feces, forming a gooey white coating around it.
Patrick: DUDE! did you hear what Lauren and Kevin did?
Andrew: No, what????
Patrick: Well Kevin took a shit on Lauren's chest then came on it and it was all gooey and white!
Andrew: Oh My God! I'm so jealous! I've always wanted to give someone a dirty snowball.
The act of a male jacking off into one hand while taking a dump in the other, then smacking his hands together, thus mixing the two together. Once together, the male balls the item up, and throws it at a womans face.
I was repulsed, yet amused when he threw a dirty snowball in my face