As used in this context, "headstrong" means obstinate, stubborn, or wilful. Capulet is scolding someone for behaving recklessly or disobediently.
In the lines from Act IV, Scene 2 of Romeo and Juliet, the term "headstrong" is used by Capulet to describe Juliet. It is used to mean stubborn, willful, or having a strong-willed nature. In this context, Capulet is expressing his frustration with Juliet's behavior and questioning where she has been wandering off to. The term "headstrong" implies that Juliet's actions and decisions are driven by her own strong determination, perhaps against what others may advise or expect.
