It was a depiction in his SciFi classic by Robt. Heinlein of a defined group within an extraterrestrial species on a distant planet. A member of this group was called a Fair Witness. The Fair Witness was incapable of reporting a falsehood. If you were out on a drive and asked a Fair Witness "What color is that house we are passing?", he might respond something like "The three sides that I have observed are white".
It's been decades since reading that book, so time may have edited that explanation.