Mulish, a.
Like a mule; sullen; stubborn. -- Mul"ish*ly, adv. -- Mul"ish*ness, n.
Shades of meaning
Fernald has no entry of its own for mulish; he lists it under obstinate, restive.
Where he writes about it:
Balky, mulish, obstinate, and stubborn are synonyms of restive only in an infrequent if not obsolete use; the supposed sense of "tending to rest," "standing stubbornly still," is scarcely supported by any examples, and those cited to support that meaning often fail to do so. — from restive
Synonyms
obstinaterestivebalkycontumaciousdecideddetermineddoggedfidgetyfirmfixedfractiousfretfulfriskyheadstrongheadyimmovableimpatientindomitableinflexibleintractablemutinousobdurateopinionatedpersistentpertinaciousrebelliousrecalcitrantrefractoryresentfulresoluteresolvedrestiffrestlessskittishstubbornunconquerableunflinchingunrulyunyieldingvicious