Source : Free On-Line Dictionary of Computing

tits on a keyboard
     
         Small bumps on certain keycaps to help touch-typists
        find the home keys (ASDF and JKL;) without looking.  They are
        nearly always on the "F" and "J" of a {QWERTY} and the "5" of
        a numeric keypad.  The {Macintosh}, perverse as usual, has
        them on the "D" and "K" keys.
     
        This term is based on the vernacular American expression "as
        useful as tits on a boar" (or boar-hog, bull, bullfrog, or
        many other variants), meaning "not useful".
     
        [{Jargon File}]
     
        (1998-02-25)