The Collaborative International Dictionary of English v.0.48:

Unsymmetrical \Un`sym*met"ric*al\, a.
   1. Wanting in symmetry, or due proportion pf parts.
      [1913 Webster]

   2. (Biol.) Not symmetrical; being without symmetry, as the
      parts of a flower when similar parts are of different size
      and shape, or when the parts of successive circles differ
      in number. See Symmetry.
      [1913 Webster]

   3. (Chem.) Being without symmetry of chemical structure or
      relation; as, an unsymmetrical carbon atom.
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   Unsymmetrical carbon atom (Chem.), one which is united at
      once to four different atoms or radicals. This condition
      usually occasions physical isomerism, with the attendant
      action on polarized light.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unsymmetrical
    adj 1: lacking symmetry [syn: unsymmetric, unsymmetrical]
    2: having unsymmetrical parts or unequal dimensions or
       measurements [syn: anisometric, unsymmetrical]