adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic).
Originally added to nouns in Old English; it was used from 13c. with verbs, and by 15c. with other adjectives (for example crispy).
Variant forms in -y for short, common adjectives (vasty, hugy) helped poets keep step with classical feet when the grammatically empty but metrically useful -e dropped off such words in late Middle English. To replace it, by Elizabethan times, verse-writers had adapted to -y forms, and often it was done artfully, as in Sackville's "The wide waste places, and the hugy plain." Simple huge plain would have been a metrical balk.
After Coleridge's criticism of the -y forms as archaic artifice, poets gave up stilly (Moore probably was last to get away with it, with "Oft in the Stilly Night"), paly (which Keats and Coleridge himself had used) and the rest. Jespersen ("Modern English Grammar," 1954) also lists bleaky (Dryden), bluey, greeny, and other color words, lanky, plumpy, stouty, and the slang rummy. Vasty survived, he said, only in imitation of Shakespeare; cooly and moisty (Chaucer, hence Spenser) he regarded as fully obsolete. But in a few cases he notes (haughty, dusky) they seem to have supplanted the shorter forms.
