The Collaborative International Dictionary of English v.0.48:

Headstrong \Head"strong`\ (-str[o^]ng`; 115), a.
   1. Not easily restrained; ungovernable; obstinate; stubborn.
      [1913 Webster]

            Now let the headstrong boy my will control.
                                                  --Dryden.
      [1913 Webster]

   2. Directed by ungovernable will, or proceeding from
      obstinacy; as, a headstrong course. --Dryden.

   Syn: Violent; obstinate; ungovernable; untractable; stubborn;
        unruly; venturesome; heady.
        [1913 Webster]

WordNet (r) 3.0 (2006):

headstrong
    adj 1: habitually disposed to disobedience and opposition [syn:
           froward, headstrong, self-willed, willful,
           wilful]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "headstrong":
   balking, balky, bigoted, bulldogged, bulletheaded, bullheaded,
   case-hardened, disobedient, dogged, dogmatic, fanatic, hardheaded,
   heady, insubordinate, intolerant, irresponsible, lawless,
   licentious, mulish, mutinous, obstinate, opinionated, overzealous,
   persevering, pertinacious, pigheaded, rampant, refractory,
   reinless, restive, self-willed, set, stiff-necked, strong-willed,
   strongheaded, stubborn, sulky, sullen, tenacious, unaccountable,
   unbridled, unchecked, uncontrolled, uncooperative, uncurbed,
   undisciplined, ungoverned, unregenerate, unreined, unrestrained,
   wildcat, willful, wrongheaded