handwave

handwave  [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.