According to Gavin Alexander, lecturer in English at Cambridge university and fellow of Milton’s alma mater, Christ’s College, who has trawled the Oxford English Dictionary (OED) for evidence, Milton is responsible for introducing some 630 words to the English language, making him the country’s greatest neologist, ahead of Ben Jonson with 558, John Donne with 342 and Shakespeare with 229. Without the great poet there would be no liturgical, debauchery, besottedly, unhealthily, padlock, dismissive, terrific, embellishing, fragrance, didactic or love-lorn. And certainly no complacency.
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Milton’s coinages can be loosely divided into five categories. A new meaning for an existing word - he was the first to use space to mean “outer space”; a new form of an existing word, by making a noun from a verb or a verb from an adjective, such as stunning and literalism; negative forms, such as unprincipled, unaccountable and irresponsible - he was especially fond of these, with 135 entries beginning with un-; new compounds, such as arch-fiend and self-delusion; and completely new words, such as pandemonium and sensuous.
Not that Milton got things all his own way. Some of his words, such as intervolve (to wind within each other) and opiniastrous (opinionated), never quite made it into regular usage - which feels like our loss rather than his.
They should just rechristen the dictionary the OEDJMAW: The Oxford English Dictionary of John Milton’s Awesome Words.
(Note to all you opiniastrous grammarians: Yes, “neologisming” is a real word. I just neologized it in honor of my new linguistic hero J. Milton.)