idioglossia \id-ee-uh-GLOS-ee-uh\, noun:
1. A private form of speech invented by one child or by children who are in close contact, as twins.
2. A pathological condition in which a person's speech is so severely distorted that it is unintelligible.
The propinquity between them had even developed into a sort of romantic idioglossia where only they knew the code words of the secret language that sooner or later is incorporated into every love affair.
-- Stuart F. Tower, The Wayfarers
Idioglossia combines the Greek roots idio-, "particular to one," and gloss-, "tongue."
2 hours ago