impulsive - adjective

• proceeding from natural feeling or impulse without external stimulus
• without forethought
• having the power of driving or impelling
• determined by chance or impulse or whim rather than by necessity or reason
• characterized by undue haste and lack of thought or deliberation

impulsive - thesaurus

brainish   capricious   driving   hotheaded   impetuous   madcap   tearaway   unprompted   whimsical