A step or flight of stairs leading downward; the lower floor or level of a building.
Compound of 'down' and 'stair,' both Old English. Originally used in singular form to describe individual descending steps, later generalized to the whole descent and lower floors.
Before 'downstairs' became standard, 'downstair' (singular) was common, showing how English naturally pluralizes or singularizes based on meaning—you can take one downstair or many downstairs.
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