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):
mulish adj 1: unreasonably rigid in the face of argument or entreaty or attack [syn: hardheaded, mulish]