Nerfgard has a rundown on what is a proc, a syntactical exercise that I think is well worth checking out.

PROCess – “Proc” is short for “process.” This is generally the most widely accepted origin of the word “proc.” Of course, you are processing a procedure (see above), so it’s entirely possible that process is derived from procedure and proc is shorthand for process. 

This is his favorite, and mine too, apparently. At least, it’s what I put down as the origin of the term in Gaming Hacks, so I guess I knew what I was talking about. For that one line. In an entire chapter.

I personally enjoy the definition of catass much better, if only for social reasons.