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2.
Fair Witness-originating from the book 'Stranger in a Strange Land' by Heinlein, Fair Witness was a made up profession, and their job was to record everything they heard/saw. What it means play/be fair witness is to, well take questions people ask you somewhat literally, as seen in the example. Yes, you can do something, but that doesn't mean you will-and that can lead into you will/would do something but not right now because no time was specified. This pretty much means messing with peoples' words/questions until they ask you in a very specific way.
James: Okay, can you pick up all of your crap?
Michael: Yes
-Waits-
James: WELL?
Michael: Oh, I'm sorry-did you want something?
James: Stop playing fair witness a pick this stuff up!
by AlliterationAnne September 04, 2009
 
1.
A "Fair Witness" is a fictional profession in the book "Stranger in a Strange Land" by Robert A. Heinlein. In the book, Fair Witnesses were individuals trained to see the world around them, as literally as possible, until sensed (see/hear/smell/taste/touch) otherwise.

Another way to describe it would be someone that takes all things sensed as literal as possible & have no reason to believe what they just sensed will remain that way when they are no longer sensing them (seeing/tasting/etc.)
An excerpt from the book itself, where the character referenced as "Anne" is an off-duty Fair Witness:

Jubal to Jill: "Even Cavendish did not--at least he won't say so. You know how Fair Witnesses behave."

Jill: "Well...no, I don't. I've never met one."

Jubal to Jill: "So? ANNE!"

Anne was on the springboard; she turned her head. Jubal called out, "That house on the hilltop--can you see what color they've painted it?"

Anne looked, then answered, "It's white on this side."

Jubal went on to Jill: "You see? It doesn't occur to Anne to infer that the other side is white, too. All the King's horses couldn't force her to commit herself...unless she went there and looked--and even then she wouldn't assume that it stayed white after she left."
by ZoLatKam March 10, 2014