What is the X in X-mas?
1800, gooroo, from Hindi guru "teacher, priest," from Sanskrit guru-s "one to be honored, teacher," literally "heavy, weighty," from PIE root *gru- (see grave (adj.)). Generalized sense of "mentor" is from 1940; sense of "expert in something" first recorded c.1966 in Canadian English in reference to Marshall McLuhan.
In Hinduism, a teacher or spiritual leader.
Note: By extension, a “guru” is a teacher who attracts disciples or followers.
[fr Sanskrit, ''venerable'']