From Google:
Dense - informal
(of a person) stupid.
synonyms: stupid, unintelligent, ignorant, brainless, mindless, foolish, slow, witless, simpleminded, empty-headed, vacuous, vapid, idiotic, imbecilic; informalthick, dim, moronic, dumb, dopey, dozy, wooden-headed, lamebrained, birdbrained, pea-brained; daft