after a post-coital affair, one steals all of the other sex partner's lubricant and previously used sex toys.
"ruthie was a dirty grinch last night. after we fucked, he stole my vibrator!"
A modification of the "Dirty Santa" gift exchange which entails picking a number and getting to steal someone's gift. The "Dirty Grinch" works the same way but you're allowed to add insult to injury by stealing the gift AND performing a Dirty Sanchez as you depart. Thereby leaving the victim with a shat mustache, no gift and green with envy.
My office had a Christmas Party and a Dirty Grinch gift exchange. I stole Sheila's gift and left her with a dirty sanchez but just like when the Grinch's heart grew three sizes that day, I paid homage by using 3 fingers. She was green with envy as I enjoyed the gift of a Jack Daniels in a pewter-studded flask.