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.