let (tennis)

Why is a nullified serve in racket sports called a let?

This term for a necessary re-serve is unrelated to the more common verb let meaning "allow."

Let in the sense of a faulty serve comes from the Old English verb lettan, "to hinder" (whereas let "allow" comes from the OE lǣtan, "to leave behind").

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acorn

Why is the fruit of an oak tree called an acorn?

Acorn derives from the Old English æcern, which meant "fruit" or "berry." Its transition from æcern to the modern-day acorn was the result of phono-semantic matching: Speakers misinterpreted æcern as oak corn, logically thinking it was called such because it was the seed (or kernel) of the oak tree (in Old English, ac = oak).

Literally: "oak kernel"

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