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):
mulishly adv 1: in a stubborn unregenerate manner; "she remained stubbornly in the same position" [syn: stubbornly, pig-headedly, obdurately, mulishly, obstinately, cussedly]