marrow (n.)

"soft tissue found in the interior of bones," late 14c., from Old English mearg "marrow," earlier mærh, from Proto-Germanic *mazga- (source also of 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" (source also of Sanskrit majjan-, Avestan mazga- "marrow," Old Church Slavonic mozgu, Lithuanian smagenės "brain"). Figurative sense of "inmost or central part, inner substance, essence" is attested from mid-14c.