Source : Free On-Line Dictionary of Computing

gorilla arm
     
        The side-effect that destroyed touch-screens as a mainstream
        input technology despite a promising start in the early 1980s.
        It seems the designers of all those {spiffy} touch-menu
        systems failed to notice that humans aren't designed to hold
        their arms in front of their faces making small motions.
        After more than a very few selections, the arm begins to feel
        sore, cramped, and oversized - the operator looks like a
        gorilla while using the touch screen and feels like one
        afterward.  This is now considered a classic cautionary tale
        to human-factors designers; "Remember the gorilla arm!" is
        shorthand for "How is this going to fly in *real* use?".