Synonyms: impetuous, hasty, headlong, precipitate
These adjectives describe abruptness or lack of deliberation. Impetuous suggests forceful impulsiveness or impatience: "[Martin Luther King] feared that an ill-prepared, impetuous demonstration would endanger ... the marchers" (Nick Kotz).
Hasty and headlong both stress hurried, often reckless action: "Hasty marriage seldom proveth well" (Shakespeare). "In his headlong flight down the circular staircase, ... [he] had pitched forward violently ... and probably broken his neck" (Mary Roberts Rinehart).
Precipitate suggests impulsiveness and lack of due reflection: "All my mistakes in life had flowed from that precipitate departure of mine" (Philip Roth).