early 13c., chateren "to twitter, make quick, shrill sounds" (of birds), "to gossip, talk idly or thoughtlessly" (of persons), earlier cheateren, chiteren, of echoic origin. Compare Dutch koeteren "jabber," Danish kvidre "twitter, chirp." Of teeth, "make a rattling noise from cold or fright," mid-15c. Related: Chattered; chattering.
Phrase chattering class was in use by 1893, with perhaps an isolated instance from 1843:
Such was the most interesting side of the fatal event to that idle chattering class of London life to whom the collision of heaven and earth were important only as affording matter for "news!" [Catherine Grace F. Gore ("Mrs. Gore"), "The Banker's Wife," 1843]
mid-13c., "a run of quick, shrill sounds," originally of birds, from chatter (v.). Meaning "idle or foolish talk" is by 1831.