This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Determined to have one's own way; stubbornly and often recklessly willful. See Synonyms at obstinate, unruly.

adj. Resulting from willfulness and obstinacy.

adj. Determined to do as one pleases, and not as others want.

adj. Not easily restrained; ungovernable; obstinate; stubborn.

adj. Directed by ungovernable will, or proceeding from obstinacy.

Wilful; ungovernable; obstinate; bent on pursuing one's own ends.

Directed by or proceeding from obstinate wilfulness: as, a headstrong course.

Synonyms Intractable, unruly, stubborn, dogged.

adj. habitually disposed to disobedience and opposition

"The only way this can be done practically, though, is if Sarah receives a rollback, a brand new rejuvenation technique that can only be afforded to the rich and powerful - like a certain headstrong millionaire."

disobedient   

headstrongly    headstrongness   

ungovernable    obstinate    stubborn    violent    heady    venturesome    unruly    untractable   

impetuous    wayward    intractable    self-willed    disobedient    quarrelsome    obstinate    capricious    imperious    undisciplined