Very similar to grimy trout. Except someone gets on hands and knees behind someone and puts their head in-between the other person's legs. Usually a male. Before splitting your buddy's legs with your cranium yell "Grimy Grizzle" and the proceed to go in-between so named fratstar's legs and move your head in a side to side motion and growl like a bear. If you feel flamboyant go up and down if you want. After doing that pull out and then stand up and say "Hey no homo though" and all is good.
"Gah i can't believe you grimy grizzled him like that. Best grizzle I've seen in a while."