impulsive

proceeding from natural feeling or impulse without external stimulus; "an impulsive gesture of affection"

without forethought; "letting him borrow her car was an impulsive act that she immediately regretted"

having the power of driving or impelling; "a driving personal ambition"; "the driving force was his innate enthusiasm"; "an impulsive force"

determined by chance or impulse or whim rather than by necessity or reason; "a capricious refusal"; "authoritarian rulers are frequently capricious"; "the victim of whimsical persecutions"

characterized by undue haste and lack of thought or deliberation; "a hotheaded decision"; "liable to such impulsive acts as hugging strangers"; "an impetuous display of spending and gambling"; "madcap escapades"; (`brainish' is archaic)