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®

mulishness
     n : the trait of being difficult to handle or overcome [syn: {stubbornness},
          {obstinacy}, {obstinance}]