in·con·stan·cy /(ˌ)ɪnˈkɑn(t)stən(t)si/
  易變,不定性,無節操,反復無常
  In·con·stan·cy n.  The quality or state of being inconstant; lack of constancy; mutability; fickleness; variableness.
  For unto knight there was no greater shame,
  Than lightness and inconstancie in love.   --Spenser.
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  inconstancy
       n 1: unfaithfulness by virtue of being unreliable or treacherous
            [syn: faithlessness, falseness, fickleness]
       2: the quality of being changeable and variable [syn: changefulness]
          [ant: constancy]