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

mouso
 /mow'soh/, n.

    [by analogy with ?typo?] An error in mouse usage resulting in an
    inappropriate selection or graphic garbage on the screen. Compare thinko,
    braino.


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

mouso

    /mow'soh/ (By analogy with "typo") An error in
   mouse usage resulting in an inappropriate selection or
   graphic garbage on the screen.

   Compare thinko.

   [Jargon File]

   (1996-04-21)