(a)dulllacking in liveliness or animation“he was so dull at parties”, “a dull political campaign”, “a large dull impassive man”, “dull days with nothing to do”, “how dull and dreary the world is”, “fell back into one of her dull moods”
(s)dull, muffled, muted, softenedbeing or made softer or less loud or clear“the dull boom of distant breaking waves”, “muffled drums”, “the muffled noises of the street”, “muted trumpets”
(s)boring, deadening, dull, ho-hum, irksome, slow, tedious, tiresome, wearisomeso lacking in interest as to cause mental weariness“a boring evening with uninteresting people”, “the deadening effect of some routine tasks”, “a dull play”, “his competent but dull performance”, “a ho-hum speaker who couldn't capture their attention”, “what an irksome task the writing of long letters is” (Edmund Burke), “tedious days on the train”, “the tiresome chirping of a cricket” (Mark Twain), “other people's dreams are dreadfully wearisome”
(s)dense, dim, dull, dumb, obtuse, slowslow to learn or understand; lacking intellectual acuity“so dense he never understands anything I say to him”, “never met anyone quite so dim”, “although dull at classical learning, at mathematics he was uncommonly quick” (Thackeray), “dumb officials make some really dumb decisions”, “he was either normally stupid or being deliberately obtuse”, “worked with the slow students”