The state of being completely free from the influence of alcohol or drugs mysteriously accompanied by a lack of desire to become intoxicated. Someone who is sober as a knob job would have no desire to drink or party and would probably go to bed at 10pm on a Saturday night.
Could also be referred to as "disgustingly sober" or "being a total fairy".
Tom: What the fuck was wrong with you last night? You wouldn't even have a beer.
Dave: I know man, for some reason I was sober as a knob job.