If you came across this word in these internet times without knowing what it means, you might guess it refers to an image whose purpose is to flag something online as stupid or witless. No such marker exists, though if it did it would surely not lack application.
It does come from the same ancient source as idiot. In Latin, an idiota was an ignorant or uneducated person, but not necessarily a fool or mentally inadequate. In classical Greek, idios referred to something private, hence idiotikos for a private person (the sense is still around in modern Greek; for example, idiotiko scholeio is a private school). Idios could also refer to somebody with his own ideas and ways of living, which survives in our idiosyncrasy and idiosyncratic.
In Greek, idiotikos could also mean ignorant or uneducated; its neuter singular idiotikon was taken into Latin after the classical period in this sense. In the eighteenth century German scholars used it for a dictionary of a dialect or a minority language — the view that they were barbarous tongues spoken only by the unschooled was still very powerful. Early examples included the Idioticon Frisicum, the Idioticon Hamburgense and the Idioticon Prussicum. Later it became a standard German word, spelled Idiotikon.
Idioticon appeared in English in the early nineteenth century in the same sense but has always been extremely rare.
I often wished for a Bronx idioticon and a Yiddish dictionary to clarify some of the words.
Sydney Morning Herald, 6 Jul. 1996.