1680s, of things, "apt to cling, adhesive," from cling + -y (2). Of persons (especially children) from 1969, though the image of a "clingy vine" in a relationship goes back to 1896. Related: Clinginess.
Old English clingan "hold fast, adhere closely; congeal, shrivel" (strong verb, past tense clang, past participle clungen), from Proto-Germanic *klingg- (source also of Danish klynge "to cluster;" Old High German klinga "narrow gorge;" Old Norse klengjask "press onward;" Danish klinke, Dutch klinken "to clench;" German Klinke "latch").
The main sense shifted in Middle English to "adhere to" (something else), "stick together." Of persons in embrace, c. 1600. Figuratively (to hopes, outmoded ideas, etc.), from 1580s. Of clothes from 1792. Related: Clung; clinging.
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; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).