I’m of the opinion that we defined it several thousand years ago, in some form at least.

https://howtobeastoic.wordpress.com/2016/01/19/one-crucial-word/

Belangia helpfully adds: “A-gnoia means literally ‘not-knowing’; a-mathia means literally ‘not-learning.’ In addition to the type of amathia that is an inability to learn, there is another form that is an unwillingness to learn. … Robert Musii in an essay called On Stupidity, distinguished between two forms of stupidity, one he called ‘an honorable kind’ due to a lack of natural ability and another, much more sinister kind, that he called ‘intelligent stupidity.'”

Belangia also quotes Glenn Hughes, from an essay entitled “Voegelin’s Use of Musil’s Concept of Intelligent Stupidity in Hitler and the Germans,” providing a further elucidation of the concept of amathia (italics in the original):

“The higher, pretentious form of stupidity stands only too often in crass opposition to [its] honorable form. It is not so much lack of intelligence as failure of intelligence, for the reason that it presumes to accomplishments to which it has no right … The stupidity this addresses is no mental illness, yet it is most lethal; a dangerous disease of the mind that endangers life itself. … [S]ince the ‘higher stupidity’ consists not in an inability to understand but in a refusal to understand, any healing or reversal of it will not occur through rational argumentation, through a greater accumulation of data and knowledge, or through experiencing new and different feelings … We may say that the reversal of a spiritual sickness must entail a spiritual cure.”