One's lifeblood or the essential, vital part of something; what you care about most deeply or what gives something its fundamental purpose.
Compounds 'heart' (Old English 'heorte') with 'blood' (Old English 'blod'). Medieval writers used this to mean the literal life-giving blood from the heart, then expanded it metaphorically to mean vital essence.
Medieval physicians thought the heart was where blood originated—totally wrong, but this misconception gave us the phrase 'heartblood' to mean something essential, showing how false science sometimes creates enduring poetry.
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