E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Down on your marrow-bones, i.e. knees. That marrow in this phrase is not a corruption of Mary, meaning the Virgin, is palpable from the analogous phrase, the marrow-bone stagewalking. The leg-bone is the marrow-bone of beef and mutton, and the play is on Marylebone (London).