secret language

They communicate in their own secret language, which scientists are beginning to decipher.

But the secret language of his heart was, "By my wisdom have I gotten all this."

The family of the Incas had a secret language of their own, which was not learned by subjects.

They took away the Queen's embroidery-work because they thought she might be sewing into it a secret language!

He was talking to the girl, not with words, but in some secret language that he and she understood alone.

It is as those among whom there is a certain secret language.