Over a decade ago, Mark Liberman posted at the Log about a garbled account of a “magical sounding gibberish language”; I won’t confuse you with the details, which turned out to be irrelevant, but the upshot was that the actual term was grammelot, which seems to have been invented by Dario Fo. A followup post has more details, and makes it clear that the notion that it is “a gibberish language … that was first described over 500 years ago” is balderdash. Mark wrote:
The confusion seems to have arisen because of Fo’s references to the 16th-century playwright Angelo Beolco. In Fo’s Nobel acceptance speech, he gave credit to “Ruzzante Beolco, my greatest master along with Molière”, called him “until Shakespeare, doubtless the greatest playwright of renaissance Europe”, and referred to the inspiration of Ruzzante’s linguistic inventiveness:
Ruzzante, the true father of the Commedia dell’Arte, also constructed a language of his own, a language of and for the theatre, based on a variety of tongues: the dialects of the Po Valley, expressions in Latin, Spanish, even German, all mixed with onomatopoeic sounds of his own invention. It is from him, from Beolco Ruzzante, that I’ve learned to free myself from conventional literary writing and to express myself with words that you can chew, with unusual sounds, with various techniques of rhythm and breathing, even with the rambling nonsense-speech of the grammelot.
(Wikipedia says that Angelo Beolco was “better known by the nickname Il Ruzzante.”) Stefano Taschini suggested that “Grammelot might result from the composition of the French words grammaire, mêler, and argot.” The exact history is murky, and if anyone knows anything more, please share. Thanks for the links go to Michael Trevor, who found out about “Grammelot” from this article on the invented penguin language Pingu, which calls Grammelot “a technique that has been used in theatre and commedia dell’arte for hundreds of years,” which may or may not be true depending on how narrowly you’re defining “technique”; at any rate, it certainly wasn’t called Grammelot.