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|Key cosmic concepts|
Guru originally referred to a spiritual teacher or guide in the eastern religions of Hinduism, Buddhism, and Sikhism. However, because of their fascination with all things "not western" many New Age doctrines and believers have co-opted the term for hucksters of their ideas.
Throughout the world many individuals have taken up the title of Guru for a wide range of practices and beliefs. Many of these Gurus create personality cults. Some of these are relatively harmless while others, such as Jim Jones, have been far more deadly.
The rationalists of India put in quite a lot of work battling their local gurus and demonstrating them to be con artists and liars.
The word is also often used colloquially to refer to someone who is an expert on something.
Basically, if the guru is just some slightly weird guy that has some good sense about something, someone who can give good tips on living or is just good at fixing things, then he is fine. If he is telling you to drink Kool-Aid mixed with bleach so that you can rise as a warrior angel for the war with the Flying Spaghetti Monster against Slenderman and his army of demonic Bronies... Either you run or you're on some kind of hallucinogen.