When you put grapes on a string and insert them into someone's asshole. That person then squeezes their ass, crushing the grapes while the first person drinks the dirty grape juice.
Mike put the string in her ass and rubbed her pussy, causing her to orgasm. She then squeezed her asshole while Mike licked upthe dirtygrape juice.
When a group of friends have a party and someone falls asleep, they place grapes into the sleeping friends ass and wait for them to rocket the grapes out when they fart. A dirty grace is when the grapes are flying.
When you shove 10 or so grapes in your butthole and cover it with grape jelly then someone tosses your salad, fruit salad in this case and you push out the grapes during the act. Provides a snack to the giver and exiting enjoyment to the recipient.
They were doing a dirty grapes in my living room and got caught, assholes ran into my bathroom an used my towels to clean up!