Marrow is the soft, fatty tissue inside bones that makes blood cells. In a more poetic sense, it can mean the deepest or most essential part of something.
From Old English 'mearg', related to German 'Mark', both meaning 'soft tissue inside bones'. The idea of an inner core led to its metaphorical use for 'essence'.
When people say something 'chilled them to the marrow', they’re imagining fear reaching the very center of their bones. Bone marrow quietly works as a blood factory, one of the most life-critical tissues we almost never see.
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