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]