Source : Webster's Revised Unabridged Dictionary (1913)

Marrow \Mar"row\, n. [OE. marou, mary, maruh, AS. mearg, mearh;
   akin to OS. marg, D. merg, G. Mark, OHG. marg, marag, Icel.
   mergr, Sw. merg, Dan. marv, Skr. majjan; cf. Skr. majj to
   sink, L. mergere. [root]274 Cf. {Merge}.]
   1. (Anat.) The tissue which fills the cavities of most bones;
      the medulla. In the larger cavities it is commonly very
      fatty, but in the smaller cavities it is much less fatty,
      and red or reddish in color.

   2. The essence; the best part.

            It takes from our achievements . . . The pith and
            marrow of our attribute.              --Shak.

   3. [OE. maru, maro; -- perh. a different word; cf. Gael.
      maraon together.] One of a pair; a match; a companion; an
      intimate associate. [Scot.]

            Chopping and changing I can not commend, With thief
            or his marrow, for fear of ill end.   --Tusser.

   {Marrow squash} (Bot.), a name given to several varieties of
      squash, esp. to the {Boston marrow}, an ovoid fruit,
      pointed at both ends, and with reddish yellow flesh, and
      to the {vegetable marrow}, a variety of an ovoid form, and
      having a soft texture and fine grain resembling marrow.

   {Spinal marrow}. (Anat.) See {Spinal cord}, under {Spinal}.

Marrow \Mar"row\, v. t. [imp. & p. p. {Marrowed}; p. pr. & vb.
   n. {Marrowing}.]
   To fill with, or as with, marrow of fat; to glut.

Source : WordNet®

marrow
     n 1: the fatty network of connective tissue that fills the
          cavities of bones [syn: {bone marrow}]
     2: any of various squash plants grown for their elongated fruit
        with smooth dark green skin and whitish flesh [syn: {marrow
        squash}, {vegetable marrow}]
     3: very tender and very nutritious tissue from marrowbones
        [syn: {bone marrow}]
     4: large elongated squash with creamy to deep green skins [syn:
         {vegetable marrow}]
     5: the choicest or most essential or most vital part of some
        idea or experience; "the gist of the prosecutor's
        argument"; "the heart and soul of the Republican Party";
        "the nub of the story" [syn: {kernel}, {substance}, {core},
         {center}, {essence}, {gist}, {heart}, {heart and soul}, {inwardness},
         {meat}, {nub}, {pith}, {sum}, {nitty-gritty}]