1800, gooroo, from Hindi guru "teacher, priest," from Skt. guru-s "one to be honored, teacher," lit. "heavy, weighty," from PIE base *gru- (see grave (adj.)). Generalized sense of "mentor" is from 1940; sense of "expert in something" first recorded c.1966 in Canadian Eng. in ref. to Marshall McLuhan.
A leader, expert, or authority in some field, esp a charismatic or spiritual figure who attracts a devoted following: turning for guidance to such gurus as Paul Goodman and Herbert Marcuse/ That genial guru of the right, Barry Goldwater(1960s+)