It would also be unsurprising to see a native speaker of classical Latin coin such a Latin word. defenestro and defenestratio are perfectly compatible with the normal methods of word formation in classical Latin, which very rarely forms compound nouns, but which forms compounds of verbs with prepositional prefixes all the time. (Just in that last sentence, you can see the ghostly remains of perfectus [thoroughly-done], compono [with-put], praepositio [before-putting], and praefixus [before-fastened], all impeccably classical. You can also see compatior [with-endure], which does not seem to have existed in classical Latin, but is obviously derived in exactly the same way as the others.)

Here ( https://www.perseus.tufts.edu/hopper/morph?l=defatigatio&la=... ) is a dictionary entry citing defatigatio to a speech given by Cicero.