The Sanskrit word derives from the proto-Indo-European root *gru which means "heavy," as in the English word "gravity." So, your guru is an imposing person, heavy with wisdom, someone whose teachings have weight, gravity.
The term guru is important in the Hindi tradition, and also in the Sikh tradition, where the word can be used to refer to a book that is a religious guide, the Guru Granth Sahib.
The image below shows Parashurama ("Rama-with-an-Axe") who was the guru to the heroes of the Mahabharata Bhishma, Drona, and Karna:
