baby: [14] Like mama and papa, baby and the contemporaneous babe are probably imitative of the burbling noises made by an infant that has not yet learned to talk. In Old English, the term for what we would now call a ‘baby’ was child, and it seems only to have been from about the 11th century that child began to extend its range to the slightly more mature age which it now covers. Then when the word baby came into the language, it was used synonymously with this developed sense of child, and only gradually came to refer to infants not yet capable of speech or walking. [baby etymology, baby origin, 英语词源]