One of the mysteries of the English language finally explained.
That may legitimately or reasonably be asked for or expected; needed, required, necessary.
Late Middle English; earliest use found in Geoffrey Chaucer (c1340–1400), poet and administrator. From require + -able. Compare Middle French requerable (of a debt, due, or payment) which needs to be claimed by the creditor, rather than being payable without a formal demand on his part.
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