The Collaborative International Dictionary of English v.0.48:

Unconstant \Un*con"stant\, a.
   Not constant; inconstant; fickle; changeable. [Obs.] --Shak.
   -- Un*con"stant*ly, adv. [Obs.] -- Un*con"stant*ness, n.
   [Obs.]
   [1913 Webster]