late 14c., from Old English mearg "marrow," earlier mærh, from Proto-Germanic *mazga- (cognates: 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" (cognates: Sanskrit majjan-, Avestan mazga- "marrow," Old Church Slavonic mozgu, Lithuanian smagenes "brain"). Figurative sense of "inmost or central part" is attested from c.1400.