Source : Free On-Line Dictionary of Computing

ohnosecond
     
         (Presumably a play on "{nanosecond}") The
        miniscule time it takes to realize that you've just made a BIG
        mistake like typing rm -rf * in the wrong directory.
     
        Seen in Elizabeth P. Crowe's book, "The Electronic Traveller."
     
        (1998-08-27)