Source : Webster's Revised Unabridged Dictionary (1913)

Blue \Blue\, a. [Compar. {Bluer}; superl. {Bluest}.] [OE. bla,
   blo, blew, blue, Sw. bl?, D. blauw, OHG. bl?o, G. blau; but
   influenced in form by F. bleu, from OHG. bl[=a]o.]
   1. Having the color of the clear sky, or a hue resembling it,
      whether lighter or darker; as, the deep, blue sea; as blue
      as a sapphire; blue violets. ``The blue firmament.''
      --Milton.

   2. Pale, without redness or glare, -- said of a flame; hence,
      of the color of burning brimstone, betokening the presence
      of ghosts or devils; as, the candle burns blue; the air
      was blue with oaths.

   3. Low in spirits; melancholy; as, to feel blue.

   4. Suited to produce low spirits; gloomy in prospect; as,
      thongs looked blue. [Colloq.]

   5. Severe or over strict in morals; gloom; as, blue and sour
      religionists; suiting one who is over strict in morals;
      inculcating an impracticable, severe, or gloomy mortality;
      as, blue laws.

   6. Literary; -- applied to women; -- an abbreviation of
      bluestocking. [Colloq.]

            The ladies were very blue and well informed.
                                                  --Thackeray.

   {Blue asbestus}. See {Crocidolite}.

   {Blue black}, of, or having, a very dark blue color, almost
      black.

   {Blue blood}. See under {Blood}.

   {Blue buck} (Zo["o]l.), a small South African antelope
      ({Cephalophus pygm[ae]us}); also applied to a larger
      species ({[AE]goceras leucoph[ae]u}s); the blaubok.

   {Blue cod} (Zo["o]l.), the buffalo cod.

   {Blue crab} (Zo["o]l.), the common edible crab of the
      Atlantic coast of the United States ({Callinectes
      hastatus}).

   {Blue curls} (Bot.), a common plant ({Trichostema
      dichotomum}), resembling pennyroyal, and hence called also
      {bastard pennyroyal}.

   {Blue devils}, apparitions supposed to be seen by persons
      suffering with {delirium tremens}; hence, very low
      spirits. ``Can Gumbo shut the hall door upon blue devils,
      or lay them all in a red sea of claret?'' --Thackeray.

   {Blue gage}. See under {Gage}, a plum.

   {Blue gum}, an Australian myrtaceous tree ({Eucalyptus
      globulus}), of the loftiest proportions, now cultivated in
      tropical and warm temperate regions for its timber, and as
      a protection against malaria. The essential oil is
      beginning to be used in medicine. The timber is very
      useful. See {Eucalyptus}.

   {Blue jack}, {Blue stone}, blue vitriol; sulphate of copper.
      

   {Blue jacket}, a man-of war's man; a sailor wearing a naval
      uniform.

   {Blue jaundice}. See under {Jaundice}.

   {Blue laws}, a name first used in the eighteenth century to
      describe certain supposititious laws of extreme rigor
      reported to have been enacted in New Haven; hence, any
      puritanical laws. [U. S.]

   {Blue light}, a composition which burns with a brilliant blue
      flame; -- used in pyrotechnics and as a night signal at
      sea, and in military operations.

   {Blue mantle} (Her.), one of the four pursuivants of the
      English college of arms; -- so called from the color of
      his official robes.

   {Blue mass}, a preparation of mercury from which is formed
      the blue pill. --McElrath.

   {Blue mold}, or mould, the blue fungus ({Aspergillus
      glaucus}) which grows on cheese. --Brande & C.

   {Blue Monday}, a Monday following a Sunday of dissipation, or
      itself given to dissipation (as the Monday before Lent).
      

   {Blue ointment} (Med.), mercurial ointment.

   {Blue Peter} (British Marine), a blue flag with a white
      square in the center, used as a signal for sailing, to
      recall boats, etc. It is a corruption of blue repeater,
      one of the British signal flags.

   {Blue pill}. (Med.)
      (a) A pill of prepared mercury, used as an aperient, etc.
      (b) Blue mass.

   {Blue ribbon}.
      (a) The ribbon worn by members of the order of the Garter;
          -- hence, a member of that order.
      (b) Anything the attainment of which is an object of great
          ambition; a distinction; a prize. ``These
          [scholarships] were the --blue ribbon of the
          college.'' --Farrar.
      (c) The distinctive badge of certain temperance or total
          abstinence organizations, as of the --Blue ribbon
          Army.

   {Blue ruin}, utter ruin; also, gin. [Eng. Slang] --Carlyle.

   {Blue spar} (Min.), azure spar; lazulite. See {Lazulite}.

   {Blue thrush} (Zo["o]l.), a European and Asiatic thrush
      ({Petrocossyphus cyaneas}).

   {Blue verditer}. See {Verditer}.

   {Blue vitriol} (Chem.), sulphate of copper, a violet blue
      crystallized salt, used in electric batteries, calico
      printing, etc.

   {Blue water}, the open ocean.

   {To look blue}, to look disheartened or dejected.

   {True blue}, genuine and thorough; not modified, nor mixed;
      not spurious; specifically, of uncompromising
      Presbyterianism, blue being the color adopted by the
      Covenanters.

Source : WordNet®

blue crab
     n 1: Atlantic crab; most common source of fresh crabmeat
     2: bluish edible crab of Atlantic and Gulf coasts of North
        America [syn: {Callinectes sapidus}]