These words refer to a lack of intellect, perceptivity or wisdom. Stupid and asinine are the harshest of these and are more likely to be used to disapprove of rather than to describe someone. Whereas stupid may suggest either a weak mentality or foolish behaviour, asinine is most often restricted to the latter: a stupid fellow; a stupid mistake; an asinine middle-aged woman dressing like a teenager. Because of this limitation, asinine is even more disapproving than stupid ; its reference is to a beast of burden, the ass, traditionally considered stupid and obstinate.

Dense and thick both concentrate on the aspect of stupid pertaining to a weak mentality, both emphasizing an inability to understand simple facts or remember clear instructions. Thick may sound more informal, but it is also less harsh than dense , which suggests an impenetrable quality of mind. Also, thick may apply to a single instance, while dense more often refers to a rooted characteristic: a bit thick ; so dense that he had to be talked to like a child.

Dumb in this sense is restricted to an extremely informal level when applied to stupid people: a dumb blonde; a dumb cluck. Purists frown on it in this use, because it means mute in other situations: the deaf and dumb ; dumb animals. Obtuse , by contrast, is perhaps the most formal word here, referring to slow-wittedness, whether revealed by a particular act or by a person’s whole character: a laughably obtuse misunderstanding of her request; an asinine television show that was specifically designed for the ignorant and obtuse . While the word may seem more objective at first glance, its very formality adds a note of withering scorn that is not present in the previous words.

Dull and unintelligent are both used more objectively to describe people or behaviour revealing low mental ability. Unintelligent, in fact, is almost completely devoid of emotional connotations: unintelligent answers to complex questions. Since mental ability is a matter of degree, the word may mask subjective judgement, except when used comparatively: gearing her lessons to the more unintelligent among her pupils. Dull can be used to express simple disapproval of the stupid and boring: The party was too dull for words; a dull -witted fool. The word is often used more objectively to refer to less than average mentality, and sometimes it serves as a euphemism for the more straightforward unintelligent . Backward is used in a similar way. Slow and retarded , too, can be regarded as euphemisms for lack of intelligence: a special class for slow learners; intellectually retarded . These terms also describe children who have been deprived of educational opportunity at some stage or who suffer from some physical disability that prevents normal progress in one or more skills.