Wiktionary
n. An in-joke; a piece of humour understood only by a group of people.
Usage examples of "private joke".
And why did he have to smile at her that way all the time, as if they were constantly sharing a secret or a very personal private joke?
He always wears a mischievous grin, as if he and Goto Dengo are sharing a little private joke.
The Simb and the Gi who were working at the think tank laughed at some private joke.
He smiled briefly at Gregor's habitual turn of phrase, a private joke.
He laughed again, warmly this time, shaking his head as if they were sharing a private joke.
He smiled as if it was some private joke only he himself could appreciate.
You'd almost have thought they were sharing a private joke-whereas in fact, I'm sure Hatsumomo was relishing her victory in finding us, and as for Mameha .