A form of the most horny dancing imaginable. Guys looking for ass will grind profusely with random girls in order to pursuade them to come home and get some bun action. Involves pushing the male genitalia up the ass of the female.
At the club last nite, I got hammered and was grinding with every girl in there...so much, that my package was sore the next day
The point during a long run when your whole body really starts to hurt and you have to really start straining to keep going. You're muscles are running out of glycogen and have to start converting fat into energy.
That 15 miler this morning turned into a real grind at about mile 12.