DramaticIdentity.

A FlatLander "knows" that there is not such direction as "up" or "down" and anyone espousing these heresies is a metaphysical blowhard.

A FlatLander demands "proof" of the existence of things that can't be rendered in his [flat] space and, seeing no such proof, declares that if it can't be rendered to his satisfaction, it doesn't exist.

''Usually used as a caricature of someone else, as a cheap attempt to shift the burden of proof.  Something like "of course there are not warp-drive powered apache helicopters, since I haven't seen any in my plane" may be signed FlatLander, implying that anyone who agrees with that statement is simply blind, and so closing off any further discussion.  YourMileageMayVary.''