I was humiliated in trying to make out the meaning of eight hard words. See how you do: claustration, edulcoration, eidolic, idoneous, infraction, straticulate, tergiversation, velleity.
The little list was included in his edition of Fowler’s Modern English Usage by the late R.W. Burchfield in 1996. He made the point that the first four of these Latinate words did not appear in the Concise Oxford Dictionary. Yet one man’s hard word is another man’s fodder for daily discourse. I wouldn’t count infraction as hard. But I failed on idoneous, which the Oxford English Dictionary (in 1899) called ‘now rare’. It means ‘fit, suitable’, as for public office, though Robert Boyle, the 17th-century scientist, used it of material suitable for producing saltpetre.
More embarrassingly, I stumbled in guessing the current meaning of two words. I thought edulcoration meant ‘sweetening’, but that sense is apparently obsolete. In the 19th century, its meaning was established as ‘the process of washing away particles soluble in water’. I doubt chemists use it much today.
I did even worse with eidolic, linking it to idols. Both eidolic and idol do come from the Greek eidolon. In Greek eidolon originally concerned images or phantoms, and to these the English word eidolic refers. Only later in Greek did it come to refer to false gods, and in this sense alone was idol first borrowed by English in the Middle Ages. In the Renaissance period, from the 16th century, idol was readopted in English with the classicised meaning of ‘an unsubstantial image’. Hence eidolic, first found in 1881.
Claustration I know from its opposite, exclaustration — temporary permission to live outside a cloistered monastery. Straticulate is an adjective to do with layers, and velleity is the supposed condition of merely wanting something while taking no action to get it. Tergiversation is now used, if at all, to mean ‘shifting, shuffling, prevarication’, rather than turning the back like an apostate or renegade.
I can’t see that it’s worse to use old hard words than hard new ones like normcore or cisgender. Merely, bear in mind that some people won’t know what you mean.