A remarkable case of survival, from Lameen at Jabal al-Lughat:
Most languages probably have a few words used especially for addressing babies. However, Siwi seems to have a lot more than I know from English or Arabic (I’ve recorded something like 40). One of these (already noted in Laoust 1931) is mbuwwa “water” (the normal Siwi word is aman). mbuwwa, meaning “water” or “drink”, turns out to be rather widespread: they use it in baby talk in Syria, Lebanon, Tunisia, Algeria, Morocco, Malta, Sicily, and probably a few other places for which I haven’t found sources. The remarkable part is that Ferguson managed to track down a historical source for this word. Varro, a Roman grammarian of the first century BC, gives bua as the nursery word for “drink” (presumably to be related to bibere, the adult verb for “drink”.) (Unfortunately, I haven’t managed to find the relevant work online.) If the connection is correct, then this word (possibly along with some others, like pappa for “bread” or “food”) has persisted in Mediterranean baby talk for at least 2000 years, apparently without ever passing into adult speech.
As several commenters point out, 40 isn’t a lot of baby-talk words (SnowLeopard says the Hopi Dictionary Project’s Hopiikwa Lavaytutuveni “lists 80 words of baby-talk Hopi”); SillyBahrainiGirl got excited (“mbuwwa! I am from Bahrain and haven’t heard this word for years!”), giving Lameen his own moment of excitement (“Wow, I didn’t realise mbuwwa went all the way from Morocco to Bahrain. I wonder if it’s used even further east?”); and the estimable bulbul got even more excited: “You have got to be kidding. Papať is the Slovak BT word for ‘to eat’, papa (feminine) means food and now you’re telling me it’s found in the Mediterranean as well? Awesome.” Yet another area of language that deserves more attention than it’s gotten to date.