Synonyms of unintelligent

1 Syllable
slow, dumb
2 Syllables
tomfool, pointless, silly, sillier, simple, foolish, witless, dense, stupid, doltish, oafish, gawkish, loopy, brainless, mindless, silliest
3 Syllables
stupidest, thickskulled, dimwitted, blockheaded, inanest, meaningless, slow-witted, nitwitted, softheaded, deficient, unwiser, senseless, inane, idiotic, blunderous, lubberly, unthinking, moronic, obtuse, half-witted, thick-headed, squirrelly
4 Syllables
loggerheaded, imbecilic, muddleheaded, empty-headed, feeble-minded, feebleminded, simpleminded, cockamamy
5 Syllables
butterfingered