1737, "deviation from the ancient and classical manner" [Johnson, who calls it "a word invented by Swift"], from modern (adj.) + -ism. From 1830 as "modern ways and styles." As a movement in the arts (away from classical or traditional modes), from 1924.
I wish you would give orders against the corruption of English by those scribblers who send us over [to Ireland] their trash in prose and verse, with abominable curtailings and quaint modernisms. [Swift to Pope, July 23, 1737]