Someone who doesn't have words to express something and they pound on the keyboard to show how excited/angry they are. Can also be used as a noun to describe the text of a keyboard spaz.
-internet chat room-
Person 1: Did you hear? Someone baked a 400 pound brownie!
Person 2: 823yu4iwhjfkfisdhsadknf;sdjfhoasuidfhsa REALLY?!!!??!!?!!?!
Person 1: Keyboard spaz much?
The keyboard spaz is not to be questioned. It just is. Much like the repeating "asdf," it's an all-purpose expression. To question it is to defeat its very purpose. The purpose being a special form of non-verbal communication. It can convey feelings of any kind.
The "asdfkasjdf;lakjsdfkjasdf" is a means of expressing one's self in ways that words cannot; especially over the internet in which words and exclamation points only hold so much meaning. As it is written in chapter three of Ecclesiastes that "there is a time for everything," there is also a time for the keyboard spaz.
In short, the meaning of the keyboard spaz is specific to its use/context, but its true meaning is far more profound. It is a communication between sender and receiver in which the receiver effortlessly intuits the intended meaning. This sacred form of communication runs deep in the intuitive connection between two like-minded people.
Friend: "OMG. Did you hear that Mr. Johnson had sex with a Freshman?"
Friend: "Right?! 'Holy shit' were my thoughts exactly!"
This, is the sacred Keyboard Spaz.