This is also known as "slide bite". It occurs when a moron fires any semi-automatic handgun and has his/her thumb too high up on the back of the weapon and when the trigger is pulled the person gets his/her thumb, or thumb region of their hand injured (usually a laceration) by the action of the slide. Glock Bite refers to when it is a Glock firearm being used by an idiot, and the idiot gets his/her hand injured.
John: Dude, lets go to the range again. I want to buck your Glock.
Joe: No numbnuts, I thought you would have had enough after that case of Glock Bite you got last time.