idioglossia
A word I learnt recently. It's defined as "a condition in which words are so poorly articulated that speech is either unintelligible or appears to be a made-up language". This is a prosaic description or explanation of the secret language some twins develop as children.
When my best friend P had his wisdom teeth removed, they gave him very strong drugs. He needed someone to look after him so I accompanied him home from the dentist's. The cabbie was a regular on the extraction run and just smiled in a knowing way when P lolled about on the back seat, mumbling. But I understood what P had said: that we had first met as baby monkeys, in the tree.