private language

noun1

1. •
   A language which expresses one's inner thoughts, feelings, or experiences but which cannot be used for communication, since it is known to and understandable by only one person—the existence of which was famously argued by Ludwig Wittgenstein (1889-1951) to be impossible.

   • But a private language—one that is spoken and understood by only one person—is a contradiction in terms. The concept of meaning is a public concept. And language is a public phenomenon. — 1996