First time I heard a frog's croak rendered as "ribbit" was in a Smothers Brothers Comedy Hour skit -- a takeoff on "The Frog Prince" -- circa 1968, the year I graduated from high school. (The point being that I never heard it as a preschool or early elementary school kid, when animal sounds are a frequent form of entertainment or education.)

I've been wondering for years whether that show was the first significant use of it.

I've tried researching it online, but found no early references. Dictionary.com doesn't list it even now. Oxford Dictionaries Online says that it's in their "U.S. English dictionary", but doesn't give an initial date. My late 1940s American College Dictionary doesn't show it (no surprise there) and (fwiw) the old 78rpm phonograph record "Tubby the Tuba", as narrated in the mid-1940s by Danny Kaye, gives a frog character a singing introduction with lots of croaking, none of which sounds even remotely like "ribbit". Wikipedia and Wiktionary are no help, though it's listed there. Ditto Urban Dictionary. Since the dictionaries characterize it as "imitative" or "onomatopoeic", there's no derivation from other languages shown.

Can anyone here help?

Eric