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late 14c., from Old English mearg "marrow," earlier mærh, from Proto-Germanic *mazga- (cf. Old Norse mergr, Old Saxon marg, Old Frisian merg, Middle Dutch march, Dutch merg, Old High German marg, German Mark "marrow"), from PIE *mozgo- "marrow" (cf. Sanskrit majjan-, Avestan mazga- "marrow," Old Church Slavonic mozgu, Lithuanian smagenes "brain"). Figurative sense of "inmost or central part" is attested from c.1400.
marrow mar·row (mār'ō)
The spinal cord.
See bone marrow.