A term, phrase, or pun that is funny to you when you say it, but not funny to anybody else. Then when somebody else says it, it is funny to them but no longer funny to you.
(pointing to a billboard advertising a bill-managing service) "Look at that BILL-board. hahaha" nobody else laughs.
Later, somebody else says "Look at that BILL-board, hahaha" and again, nobody else laughs not even the original joker.
This situation is a keyosk.