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.