The shredding cut just below the thumb one receives from attempting to twist-open a beer that requires a bottle opener. Beer wounds typically occur in direct proportion to how drunk the would-be consumer is.
Gary: Fuck man, what happened to your hand?
Greg: Last night I was so hammered I spent 15 minutes trying to twist-open a Hoegaarden. Turns out you need a bottle opener for those...
Gary: Well, scored yourself a grade A beer wound!