Source : Webster's Revised Unabridged Dictionary (1913)

Marrowbone \Mar"row*bone`\, n.
   A bone containing marrow; pl. ludicrously, knee bones or
   knees; as, to get down on one's marrowbones, i. e., to kneel.

Source : WordNet®

marrowbone
     n : a bone containing edible marrow; used especially in
         flavoring soup