The definition given this word by its inventor, the nineteenth-century Italian criminologist Cesare Lombroso was “semi-insane”. He believed that criminality was inherited and that a criminal was born with physical defects identifying him as a degenerate human being, an atavism. He created mattoid from the Italian matto, insane, plus the ending -oid for some likeness or resemblance (from Greek eidos, form). He used it for what psychiatrists call “borderline dwellers”, those who exist on the margins between reason and madness — in everyday speech we might call them cranks, eccentrics, or misfits.
The word came into English in 1891 through a translation of his work Man of Genius and became popular for a while. H G Wells used it in several of his books, most notably in Mankind in the Making of 1903, in which he derides the theories of Lombroso and the Victorian phrenologists: “Among such theorists none at present are in quite such urgent need of polemical suppression as those who would persuade the heedless general reader that every social failure is necessarily a ‘degenerate’, and who claim boldly that they can trace a distinctly evil and mischievous strain in that unfortunate miscellany which constitutes ‘the criminal class’... These mattoid scientists make a direct and disastrous attack upon the latent self-respect of criminals.”