In order to have a clear order of precedence I looked for the ur-meanings of my terms, and the ever faithful wikipedia came through under the Idiot sub-heading of Disability:
Individuals with the lowest mental age level (less than three years) were identified as idiots; imbeciles had a mental age of three to seven years, and morons had a mental age of seven to ten years.This is reasonably close to how I would classify actual children of those various ages so the glove fits nicely. The entry also mentions that Idiots are barred from voting in Kentucky, Mississippi, New Mexico, Ohio, and British parliamentary elections. Would it were so...
From this I have developed my idiosyncratic taxonomy for use in further discussions.
An Idiot does stupid and or dangerous things over and over. In current American medical classification, these people are now said to have profound intellectual disability.
A Dunce is an idiot who is specifically incapable of learning.
An Imbecile has an intellectual disability less extreme than idiocy. This is now usually subdivided into two categories, known as severe intellectual disability and moderate intellectual disability (again, thank you wikipedia). I apply this word to media personalities who just make up information and then get huffy when someone argues with them.
A Moron misses connections which are staring him/her right in the face. They do however usually have the ability to learn once something is explicitly explained to them (Brian, and now I, pretty clearly fit in this category).
"Moron" was coined in 1910 by psychologist Henry H. Goddard from the Ancient Greek word μωρός (moros), which meant "dull" (as opposed to oxy, which meant "sharp" (see also: oxymoron)) [!emphasis mine!] [wikipedia:Moron]
Then the various categories which reflect lack of experience, education, or tact rather than missing capability, and thus may be redeemable.
A Fool is unwise or lacking in judgement.
An Ignoramus is stupid, uneducated or ignorant.
A Dolt is stupid and entirely tedious at the same time. According to the Urban Dictionary they may also be oblivious to their own mental incapacity.
A Dullard is an unimaginative person. Again with thanks to The UD: An omnipresent, boring, annoying, and frequently idiotic being who is a master of inane conversations. Note that Websters has an extensive set of synonyms under dullard, but only two antonyms...
A Cretin is a vulgar or insensitive person, c.f.: clod, lout. A special sub-class (which may not be redeemable) was created by Karl Marx and best described by Friedrich Engels:
Parliamentary cretinism, an incurable disease, an ailment whose unfortunate victims are permeated by the lofty conviction that the whole world, its history and its future are directed and determined by a majority of votes of just that very representative institution that has the honour of having them in the capacity of its members. [Encyclopedia of Marxism]Working our way further into politics:
A Jackass will do anything that might make them seem to be popular. I apply this to most politicians.
An Asshat has their head up their ass, or more technically:
Their cranial capacity has been reified by the hegemony of the interiority of their posterior. [Schippling, 2014]Asshats believe what imbeciles (media) and jackasses (politicians) say.
A further set of categories applies to working environments:
Dweebs are people that aren't really capable of anything but sometimes try anyway. Most employees throughout the management hierarchy are dweebs.
Then there are the Nerds and Geeks. They're pretty much the same except for one important quality. Both Nerds and Geeks are fairly tightly focused and capable in some, usually technological, subject. The difference is that the nerd thinks he actually enjoys doing what he's doing. The geek knows better.
Yahoos are egregious people whom you can't live without because they get things get done.
And finally, Gurus can be truculent and unpredictable. They amuse themselves by solving obscure problems and, if you are lucky, one of those problems will be yours.
Many of these folks are intelligent enough to believe that they know everything. Some few realize that they don't. An even smaller number actually do know everything, or at least how to figure it out.