guru (n.)

1806, gooroo, from Hindi guru "teacher, priest," from Sanskrit guru-s "one to be honored, teacher," from guru- "venerable, worthy of honor," literally "heavy, weighty," from PIE root *gwere- (1) "heavy." Generalized sense of "mentor" is from 1940 (in H.G. Wells); sense of "expert in something" first recorded c. 1966 in Canadian English in reference to Marshall McLuhan.