A sexual act involving two people, an apple, and gravity. Participant A places an apple in their anus, while participant B takes up position below participant A's anus. Participant A then attempts to hit person B on the head with the apple. In some variations, participant B will attempt to catch the apple in his mouth, though this is not a classical Dirty Isaac Newton.
I had this girl once who loved dirty Isaac Newtons. She even ate the apple afterwards.
by Haje June 09, 2010