If you describe someone as impetuous, you mean that they are likely to act quickly and suddenly without thinking or being careful. He was young and impetuous = rash, impulsive. tending to do things very quickly, without thinking carefully first, or showing this quality = impulsive (impétueux, from impetuosus, from impetus; IMPETUS)
Describing one who makes arbitrary decisions, esp. in an impulsive and forceful manner
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)
Rushing with force and violence; moving with impetus; furious; forcible; violent; as, an impetuous wind; an impetuous torrent