The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   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.
      [1913 Webster]

   2. The essence; the best part.
      [1913 Webster]

            It takes from our achievements . . .
            The pith and marrow of our attribute. --Shak.
      [1913 Webster]

   3. [OE. maru, maro; -- perh. a different word; cf. Gael.
      maraon together.] One of a pair; a match; a companion; an
      intimate associate. [Scot.]
      [1913 Webster]

            Chopping and changing I can not commend,
            With thief or his marrow, for fear of ill end.
                                                  --Tusser.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

marrow squash
    n 1: any of various squash plants grown for their elongated
         fruit with smooth dark green skin and whitish flesh [syn:
         marrow, marrow squash, vegetable marrow]