marrowbone (n.)

late 14c., marybones (late 13c. as a surname), "bone containing fat or marrow," from marrow + bone (n.). A poetic Old English word for "bone" was mearhcofa "marrow-chamber." Later generally of any large bone. The conjecture that it is a corruption of Mary-bones, in allusion to the reverence paid to the Virgin Mary by kneeling "is absurd" [Century Dictionary]; nonetheless, marrowbones is used especially to mean "the bones of the knees" (1530s). To ride in the marrow-bone coach was one of many terms in old slang for "to go on foot."