Historical note: One correspondent believes `chad' (sense 2) derives from the Chadless keypunch (named for its inventor), which cut little u-shaped tabs in the card to make a hole when the tab folded back, rather than punching out a circle/rectangle; it was clear that if the Chadless keypunch didn't make them, then the stuff that other keypunches made had to be `chad'. There is a legend that the word was originally acronymic, standing for "Card Hole Aggregate Debris", but this has all the earmarks of a bogus folk etymology.