1. [n]  an embarrassing mistake
2. [v]  make an error
3. [v]  commit a faux pas or fault

BLUNDER is a 7 letter word that starts with B.

1. \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.
   
   2. To move in an awkward, clumsy manner; to flounder and
      stumble.
   
            I was never distinguished for address, and have
            often even blundered in making my bow. --Goldsmith.
   
            Yet knows not how to find the uncertain place, And
            blunders on, and staggers every pace. --Dryden.
   
   {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.
   
   

2. \Blun"der\, v. t.
   1. To cause to blunder. [Obs.] ``To blunder an adversary.''
      --Ditton.
   
   2. To do or treat in a blundering manner; to confuse.
   
            He blunders and confounds all these together.
                                                  --Stillingfleet.
   
   

3. \Blun"der\, n.
   1. Confusion; disturbance. [Obs.]
   
   2. A gross error or mistake, resulting from carelessness,
      stupidity, or culpable ignorance.
   
   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.