The Collaborative International Dictionary of English v.0.48:
Mulish \Mul"ish\ (m[=u]l"[i^]sh), a. Like a mule; sullen; stubborn. -- Mul"ish*ly, adv. -- Mul"ish*ness, n. [1913 Webster]WordNet (r) 3.0 (2006):
mulishness n 1: the trait of being difficult to handle or overcome [syn: stubbornness, obstinacy, obstinance, mulishness]