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]