My partner hands me a coffee. "Daydoo," I say absently. She understands - without even realizing that I am not speaking English - and responds, "Weggum." We have made no conscious choice to start speaking the idiolect of our two-year-old, but his language seeps into ours. If you live in France, you accidentally start speaking French. If someone very close to you says "toobee" for "excuse me" (and particularly if that person usually says "toobee, daddy," in an impossibly cute way), and everyone in your little house - a world of its own - understands what "toobee" means, you may be charmed into thinking that "toobee" is the preferable formula for moving someone out of your way.
This is the opposite of what speech and language therapists counsel you to do, of course: You should repeat the garbled phrase in clear English so that the child will learn to correct his eccentric ways. One tries to do this, but the natural instinct to mimic one's interlocutor runs both ways. This human tendency is surely crucial to the development of all communication: We adjust to and repeat the speech patterns of others in order to make them feel at ease.
Furthermore, there is a great emotional and egotistical appeal to secret languages - languages spoken only between lovers, for example. We all love code words. And is it a coincidence that lovers' talk is so frequently baby talk? Our speaking two-year-old dialect is a form of bonding. So now I call out in the supermarket, unconsciously, that I am going to get a carton of "mowkie" and that we might have "pida" for lunch, and I don't care who hears it.
Sometimes my son's invented words have no resemblance to English words - they are entirely his. Since he was about 16 months old, "ubbadeebee" has clearly meant both pick me up and put me down. (Also put me in my high chair or remove me from it.) I have no idea where it came from (maybe a daycare worker saying "upsy-daisy" at a formative stage?), but it has always been clearly enunciated. As there is no English word that so neatly summarizes both this action and its converse, both parents now ask, "Ubbadeebee?" instead of "Would you like to get up/down?" His Polish-speaking grandparents also use this word with him as it crosses languages. (I know, all you language therapists - wrong, wrong, wrong!)
Interestingly, linguists argue over exactly what constitutes an idiolect, or purely personal language, because of course we all have our own styles of speech and favoured idioms; we each speak in a unique dialect, and there is no ideal language that exists outside our use of it. When a child speaks a really unusual, totally private language - frequently with a twin - it's called idioglossia or cryptophasia. When it's a stream of syllables without any real pattern or meaning, as in religious "speaking in tongues," it's called glossolalia.
All these variants on common languages are useful in art as a way of expanding expression. Poets have spoken gibberish since Dada at least (and even before - one might argue that bits of Mallarmé or Shelley are hardly coherent). Finnegans Wake is written in a distinctive idiolect. The singers of the eighties pop groups Dead Can Dance and Cocteau Twins used idioglossias in their songs to great atmospheric effect.
In an early story, John Updike describes a two-year-old's proto-words this way: "Language is to him thick vague handles swirling by; he grabs what he can."
I love the image - particularly for its appropriately childlike suggestion of a merry-go-round - but I see the process through a different metaphor. I see my son's words as rough marble blocks from which he has begun to chip away, to define the word buried within. He will get closer and closer in his pronunciation to the sharply defined sculpture that is in each one. This process will be slow or fast, but it will end up in adult language. In the meantime, the impressionist sculptures he does manage are eloquent in their very muddiness; they are thick and meaty and colourful like any semi-abstract art about which you might think, "My two-year-old could do that!"