Noun
not the sharpest knife in the drawer
(humorous, euphemistic) Lacking in intelligence; dimwitted.
Synonyms: not the brightest bulb in the chandelier, not the sharpest crayon in the box, not the sharpest tool in the box, not the sharpest tool in the shed, dumb as a bag of hammers, dumb as a box of rocks; see also Thesaurus:stupid