The Collaborative International Dictionary of English v.0.48:

Demean \De*mean"\, v. t. [imp. & p. p. Demeaned; p. pr. & vb.
   n. Demeaning.] [OF. demener to conduct, guide, manage, F.
   se d['e]mener to struggle; pref. d['e]- (L. de) + mener to
   lead, drive, carry on, conduct, fr. L. minare to drive
   animals by threatening cries, fr. minari to threaten. See
   Menace.]
   1. To manage; to conduct; to treat.
      [1913 Webster]

            [Our] clergy have with violence demeaned the matter.
                                                  --Milton.
      [1913 Webster]

   2. To conduct; to behave; to comport; -- followed by the
      reflexive pronoun.
      [1913 Webster]

            They have demeaned themselves
            Like men born to renown by life or death. --Shak.
      [1913 Webster]

            They answered . . . that they should demean
            themselves according to their instructions.
                                                  --Clarendon.
      [1913 Webster]

   3. To debase; to lower; to degrade; -- followed by the
      reflexive pronoun.
      [1913 Webster]

            Her son would demean himself by a marriage with an
            artist's daughter.                    --Thackeray.
      [1913 Webster]

   Note: This sense is probably due to a false etymology which
         regarded the word as connected with the adjective mean.
         [1913 Webster]