The Collaborative International Dictionary of English v.0.48:

Bullheaded \Bull"head`ed\ (b[.u]l"h[e^]d`[e^]d), a.
   Having a head like that of a bull. Fig.: Headstrong;
   obstinate; dogged.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bullheaded
    adj 1: obstinate and stupid [syn: bullheaded, bullet-headed,
           pigheaded]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "bullheaded":
   adherent, adhesive, balking, balky, bigoted, bulldogged,
   bulldoggish, bulldoggy, bulletheaded, case-hardened, clingy,
   closed-minded, deaf, dogged, dogmatic, fanatic, gluey, glutinous,
   gummy, hardheaded, headstrong, intolerant, intractable, mulish,
   obstinate, opinionated, overzealous, persevering, persistent,
   pertinacious, perverse, pigheaded, refractory, restive,
   self-adhesive, self-willed, set, sot, stickable, sticky,
   stiff-necked, strong-willed, strongheaded, stubborn, sulky, sullen,
   tacky, tenacious, tough, uncooperative, unregenerate, viscid,
   willful