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.