Source : Webster's Revised Unabridged Dictionary (1913)

Mulish \Mul"ish\ (m[=u]l"[i^]sh), a.
   Like a mule; sullen; stubborn. -- {Mul"ish*ly}, adv. --
   {Mul"ish*ness}, n.

Source : WordNet®

mulish
     adj : unreasonably rigid in the face of argument or entreaty or
           attack [syn: {hardheaded}]