The Collaborative International Dictionary of English v.0.48:

Rash \Rash\, a. [Compar. Rasher (-[~e]r); superl. Rashest.]
   [Probably of Scand. origin; cf. Dan. & Sw. rask quick, brisk,
   rash, Icel. r["o]skr vigorous, brave, akin to D. & G. rasch
   quick, of uncertain origin.]
   1. Sudden in action; quick; hasty. [Obs.] "Strong as aconitum
      or rash gunpowder." --Shak.
      [1913 Webster]

   2. Requiring sudden action; pressing; urgent. [Obs.]
      [1913 Webster]

            I scarce have leisure to salute you,
            My matter is so rash.                 --Shak.
      [1913 Webster]

   3. Esp., overhasty in counsel or action; precipitate;
      resolving or entering on a project or measure without due
      deliberation and caution; opposed to prudent; said of
      persons; as, a rash statesman or commander.
      [1913 Webster]

   4. Uttered or undertaken with too much haste or too little
      reflection; as, rash words; rash measures.
      [1913 Webster]

   5. So dry as to fall out of the ear with handling, as corn.
      [Prov. Eng.] --Grose.
      [1913 Webster]

   Syn: Precipitate; headlong; headstrong; foolhardy; hasty;
        indiscreet; heedless; thoughtless; incautious; careless;
        inconsiderate; unwary.

   Usage: Rash, Adventurous, Foolhardy. A man is
          adventurous who incurs risk or hazard from a love of
          the arduous and the bold. A man is rash who does it
          from the mere impulse of his feelings, without
          counting the cost. A man is foolhardy who throws
          himself into danger in disregard or defiance of the
          consequences.
          [1913 Webster]

                Was never known a more adventurous knight.
                                                  --Dryden.
          [1913 Webster]

                Her rash hand in evil hour
                Forth reaching to the fruit, she plucked, she
                eat.                              --Milton.
          [1913 Webster]

                If any yet be so foolhardy
                To expose themselves to vain jeopardy;
                If they come wounded off, and lame,
                No honor's got by such a maim.    --Hudibras.
          [1913 Webster]