Plural of calf, referring either to young cattle or the muscular back part of the human leg below the knee. The context determines which meaning applies.
From Old English 'cealf' for the young animal, and a separate development for the leg muscle. The leg sense comes from the resemblance of the muscle's shape to a young calf's plump form.
It's remarkable that the same word describes both baby cows and leg muscles purely by visual analogy - ancient people noticed the rounded, swollen appearance of calf muscles resembled the plump form of young cattle. This dual meaning exists in many languages, suggesting the comparison was quite universal among agricultural societies.
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