In its earlier days, infant was sometimes spelled infaunt, and it also sometimes meant "fetus". For the most part, however, it retained its modern spelling and definition, with very little variation in form and frequency of usage. This comes directly from the Latin nominative infans, which literally meant "not speaking", literally because infants cannot speak. Although it kind of makes sense, it's pretty surprising nonetheless. If we break up this work, we can separate the prefix in-, meaning "not", and the root fans, which is a conjugation of the verb for, meaning "to speak". Via Proto-Italic en, in- derives from the Proto-Indo-European reconstruction n, which also meant "not". Meanwhile, for came by way of Proto-Italic faor from Proto-Indo-European bheh, or "to speak" as well. So, not much semantic development, but a very interesting hidden definition.
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