a knock is someone who occasionally does impulsive, immature, random things (such as buying a 12-pack of mountain dew game fuel). Knocks are basically people who don't care what other people think of them, and act like idiots.
There are 2 kinds of knocks: knocks that are just joking around, or knocks who are actually complete idiots. This word has been used to mean various things, but this use of the word was invented in Berkeley, CA, and brought to widespread use by students at berkeley high school
"dude!! I just downloaded 2 Andrew W.K. albums!"
"you are such a knock!!"