The Collaborative International Dictionary of English v.0.48:

Muss \Muss\, v. t. [imp. & p. p. Mussed; p. pr. & vb. n.
   Mussing.]
   To disarrange, as clothing; to rumple; -- often used with up;
   as, the wind mussed up my hair. [Colloq. U.S.]
   [1913 Webster]