Do you know ghouls from goblins and ghosts?
c.1300, pur blind "entirely blind," as a noun, "a blind person," later "partially blind, blind in one eye" (late 14c.), the main modern sense, from blind (adj.). The first element is sometimes explained as pure (adj.), or as the Anglo-French perfective prefix pur- (see pur-). Sense of "dull" first recorded 1530s.
purblind pur·blind (pûr'blīnd')
Having poor vision; nearly or partly blind.
Slow in understanding or discernment; dull.