The Jargon File (version 4.4.7, 29 Dec 2003):

handwave
 /hand'wayv/

    [poss. from gestures characteristic of stage magicians]

    1. v. To gloss over a complex point; to distract a listener; to support a
    (possibly actually valid) point with blatantly faulty logic.

    2. n. The act of handwaving. ?Boy, what a handwave!?

    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 bogus. 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.


The Free On-line Dictionary of Computing (30 December 2018):

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]