The Collaborative International Dictionary of English v.0.48:

Weasel \Wea"sel\, n. [OE. wesele, AS. wesle; akin to D. wezel,
   G. wiesel, OHG. wisala, Icel. hreyiv[imac]sla, Dan.
   v[aum]sel, Sw. vessla; of uncertain origin; cf. Gr. ?, ?,
   cat, weasel.] (Zool.)
   Any one of various species of small carnivores belonging to
   the genus Putorius, as the ermine and ferret. They have a
   slender, elongated body, and are noted for the quickness of
   their movements and for their bloodthirsty habit in
   destroying poultry, rats, etc. The ermine and some other
   species are brown in summer, and turn white in winter; others
   are brown at all seasons.
   [1913 Webster]

   Malacca weasel, the rasse.

   Weasel coot, a female or young male of the smew; -- so
      called from the resemblance of the head to that of a
      weasel. Called also weasel duck.

   Weasel lemur, a short-tailed lemur (Lepilemur
      mustelinus). It is reddish brown above, grayish brown
      below, with the throat white.
      [1913 Webster]

WordNet (r) 3.0 (2006):

weasel
    n 1: a person who is regarded as treacherous or sneaky
    2: small carnivorous mammal with short legs and elongated body
       and neck

The Free On-line Dictionary of Computing (19 January 2023):

weasel

    (Cambridge) A naive user, one who deliberately
   or accidentally does things that are stupid or ill-advised.
   Roughly synonymous with loser.

   [Jargon File]

   (1995-03-21)


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

weasel
 n.

    [Cambridge] A naive user, one who deliberately or accidentally does things
    that are stupid or ill-advised. Roughly synonymous with loser.