A facepalmist is someone, who by virtue one's idiocy (unaccountable by a disease or disorder), gives the reader/listener an inevitable, painful collision between one's palm and the face, one of the reasons being the utter disgust of having to share the identity of "human" with the facepalmist.
This person is not the regular ignorant person (which is basically each one of us) who says something stupid once in a while. Both the quantity and quality of stupid matters, he/she has to be very stupid and often stupid. The suffix "ist" is justified by the expert level stupidity in a facepalmist.
Facepalmist: If some stars are 100 light years away, then how do I see them? I am not 10 million years old, stupid cosmologists!
Anyone with basic common sense: *Facepalm*