In the direction toward a lower place or level; moving from a higher position to a lower one.
From Old English 'dun' or 'doun' (hill, mountain, meaning low), combined with 'weard' (direction, way). The phrase literally meant 'in the direction of lower ground.'
The word 'down' is so old that it originally meant 'from the hills'—when you went 'down' from higher elevation, you were literally moving from the downs (hills) toward lower ground, so the direction became the word!
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