After a night of excessive drinking away from home, a Grizzly Bear Attack occurs when you black out your walk home and wake up covered in scratches, cuts, bruises, etc. with no reasonable explanation. You often lose your wallet or pants in a Grizzly Bear Attack.
Jeff: Dude, I feel like shit! I'm so sore!
Joe: What happened last night?
Jeff: I have no idea, must have been a Grizzly Bear Attack on my walk home