mid-15c., "talk idly, babble," short for chatter (v.). Meaning "to converse familiarly" is from 1550s. Sense of "flirt with, ingratiate oneself with" (later often with up (adv.)) is from 1898. Related: Chatted; chatting.
1520s, "chatter, frivolous talk;" see chat (v.). Meaning "familiar conversation" is from 1570s. As a name for birds with chattering cries, 1690s. Chat show for what in U.S. is a talk show is attested from 1967. Chat room in the online sense is attested by 1994, from the days when AOL ruled the World Wide Web.