The Collaborative International Dictionary of English v.0.48:

Blunder \Blun"der\, v. t.
   1. To cause to blunder. [Obs.] "To blunder an adversary."
      --Ditton.
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   2. To do or treat in a blundering manner; to confuse.
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            He blunders and confounds all these together.
                                                  --Stillingfleet.
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The Collaborative International Dictionary of English v.0.48:

Blunder \Blun"der\, n.
   1. Confusion; disturbance. [Obs.]
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   2. A gross error or mistake, resulting from carelessness,
      stupidity, or culpable ignorance.
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   Syn: Blunder, Error, Mistake, Bull.

   Usage: An error is a departure or deviation from that which
          is right or correct; as, an error of the press; an
          error of judgment. A mistake is the interchange or
          taking of one thing for another, through haste,
          inadvertence, etc.; as, a careless mistake. A blunder
          is a mistake or error of a gross kind. It supposes a
          person to flounder on in his course, from
          carelessness, ignorance, or stupidity. A bull is a
          verbal blunder containing a laughable incongruity of
          ideas.
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The Collaborative International Dictionary of English v.0.48:

Blunder \Blun"der\, v. i. [imp. & p. p. Blundered; p. pr. &
   vb. n. Blundering.] [OE. blunderen, blondren, to stir,
   confuse, blunder; perh. allied to blend to mix, to confound
   by mixture.]
   1. To make a gross error or mistake; as, to blunder in
      writing or preparing a medical prescription. --Swift.
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   2. To move in an awkward, clumsy manner; to flounder and
      stumble.
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            I was never distinguished for address, and have
            often even blundered in making my bow. --Goldsmith.
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            Yet knows not how to find the uncertain place,
            And blunders on, and staggers every pace. --Dryden.
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   To blunder on.
      (a) To continue blundering.
      (b) To find or reach as if by an accident involving more
          or less stupidity, -- applied to something desirable;
          as, to blunder on a useful discovery.
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WordNet (r) 3.0 (2006):

blunder
    n 1: an embarrassing mistake [syn: blunder, blooper,
         bloomer, bungle, pratfall, foul-up, fuckup,
         flub, botch, boner, boo-boo]
    v 1: commit a faux pas or a fault or make a serious mistake; "I
         blundered during the job interview" [syn: drop the ball,
         sin, blunder, boob, goof]
    2: make one's way clumsily or blindly; "He fumbled towards the
       door" [syn: fumble, blunder]
    3: utter impulsively; "He blurted out the secret"; "He blundered
       his stupid ideas" [syn: blurt out, blurt, blunder out,
       blunder, ejaculate]