infantyoudaoicibaDictYouDict[infant 词源字典]
infant: [14] Etymologically, an infant is ‘someone who cannot yet speak’. The word comes via Old French enfant from Latin infāns ‘young child’, a noun use of the adjective infāns, originally ‘unable to speak’, which was formed from the negative prefix in- and the present participle of fārī ‘speak’ (source of English fable, fame, fate, etc).

The somewhat improbable derivative infantry [16] comes via French from Italian infanteria; this was based on infante, whose original meaning ‘young person’ had shifted to ‘foot soldier’ (a development distantly reminiscent of the use of British English lads for ‘male members of a group, team, etc’).
=> fable, fame, fate[infant etymology, infant origin, 英语词源]
infant (n.)youdaoicibaDictYouDict
late 14c., "child during earliest period of life" (sometimes extended to age 7 and sometimes including a fetus), from Latin infantem (nominative infans) "young child, babe in arms," noun use of adjective meaning "not able to speak," from in- "not, opposite of" (see in- (1)) + fans, present participle of fari "to speak," from PIE root *bha- (2) "to speak, tell, say" (see fame (n.)). As an adjective, 1580s, from the noun.