Source : Free On-Line Dictionary of Computing

handwave
     
        [possibly from gestures characteristic of stage magicians] To
        gloss over a complex point; to distract a listener; to support
        a (possibly actually valid) point with blatantly faulty logic.
     
        If someone starts a sentence with "Clearly..." or
        "Obviously..." or "It is self-evident that...", it is a good
        bet he is about to handwave (alternatively, use of these
        constructions in a sarcastic tone before a paraphrase of
        someone else's argument suggests that it is a handwave).  The
        theory behind this term is that if you wave your hands at the
        right moment, the listener may be sufficiently distracted to
        not notice that what you have said is wrong.  Failing that, if
        a listener does object, you might try to dismiss the objection
        with a wave of your hand.
     
        The use of this word is often accompanied by gestures: both
        hands up, palms forward, swinging the hands in a vertical
        plane pivoting at the elbows and/or shoulders (depending on
        the magnitude of the handwave); alternatively, holding the
        forearms in one position while rotating the hands at the wrist
        to make them flutter.  In context, the gestures alone can
        suffice as a remark; if a speaker makes an outrageously
        unsupported assumption, you might simply wave your hands in
        this way, as an accusation, far more eloquent than words could
        express, that his logic is faulty.
     
        [{Jargon File}]