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)
without forethought; "letting him borrow her car was an impulsive act that she immediately regretted" proceeding from natural feeling or impulse without external stimulus; "an impulsive gesture of affection
If you describe someone as impulsive, you mean that they do things suddenly without thinking about them carefully first. He is too impulsive to be a responsible prime minister + impulsively im·pul·sive·ly He studied her face for a moment, then said impulsively: `Let's get married' + impulsiveness im·pul·sive·ness The president's impulsiveness often worries his advisers. someone who is impulsive does things without considering the possible dangers or problems first