Not naive; more experienced than one might think. Already a popular saying by the early nineteenth century, it appeared on both sides of the Atlanti “I warn’t born yesterday,” said Thomas Haliburton’s Sam Slick in one of his Wise Saws (1843). Approximately a century later Garson Kanin used the phrase for the title of a Broadway play that became extremely popular, as did the later (1950) film version. In both, actress Judy Holliday played the quintessential dumb blonde who, despite seeming unsophistication, is graced with enormous good sense.

Extremely old-fashioned. The “so” in this phrase, which dates from the late 1900s, means “extremely” or “completely.” An article about extending human life indefinitely was headlined “Merely Human? That’s So Yesterday” (New York Times, Ashlee Vance, June 13, 2010). And another, from the Globe and Mail (Montreal), had “The Traditional Job Interview: That’s So Yesterday” (Craig Silverman, April 7, 2008). For an older synonym, see old hat.