Sub being short for subwoofers in the trunk. When someone pulls up beside your vehicle with their music blaring and you don't want to look weak so you turn yours up too.
John: You mean SUB-Urban Dictionary? Maybe, but I'd have to filter through a lot of crap to find any terms worth using. I'm better off trawling for words on a web-forum!
A level of quality or awesomeness that is significantly lower than the highest level of quality or awesomeness, or in the most common cases, completely lacking in quality or awesomeness.
SWDev A: Are those guys from specs smoking crack or something? This is a fricking joke!
SWDev B: Well, I will admit the latest revision certainly is sub-awesome.