Thank you! for providing me with the opportunity to learn. I was surprised by your response in the way that I had always thought of ‘guru’ as teacher. Now I realize that it is more of a Hindu expression, which has no relevance here. But I did find this from the Free Dictionary:
Guru (Devanagari गुरु) is a Sanskrit term for “teacher” or “master”, especially in Indian religions. The Hindu guru-shishya tradition is the oral tradition or religious doctrine or experiential wisdom transmitted from teacher to student. In the United States, the meaning of “guru” has been used to cover anyone who acquires followers, especially by exploiting their naiveté, due to the inflationary use of the term in new religious movements.
The syllable gu means shadows
The syllable ru, he who disperses them,
Because of the power to disperse darkness
the guru is thus named.
— Advayataraka Upanishad 14—18, verse 5
The word guru, a noun, means “teacher” in Sanskrit and in other languages derived from or borrowing words from Sanskrit, such as Hindi, Punjabi, Tamil, Telugu, Kannada, Malayalam, Marathi, Oriya, Bengali, Gujarati and Nepali. The Malayalam term Acharyan or Asan are derived from the Sanskrit word Acharya. It is transliterated in different ways such as “Asaan”, “Ashan”, “Aasaan” etc.
As a noun the word means the imparter of knowledge (jñāna; also Pali: ñāna). As an adjective, it means ‘heavy,’ or ‘weighty,’ in the sense of “heavy with knowledge,” heavy with spiritual wisdom, “heavy with spiritual weight,” “heavy with the good qualities of scriptures and realization,” or “heavy with a wealth of knowledge.” The word has its roots in the Sanskrit gri (to invoke, or to praise), and may have a connection to the word gur, meaning ‘to raise, lift up, or to make an effort’.
Sanskrit guru is cognate with Latin gravis ‘heavy; grave, weighty, serious’ and Greek βαρύς barus ‘heavy’. All Proto-Indo-European root *gʷerə-, specifically from the zero-grade form *gʷr̥ə-.
A traditional etymology of the term “guru” is based on the interplay between darkness and light. The guru is seen as the one who “dispels the darkness of ignorance.” In some texts it is described that the syllables gu (गु) and ru (रु) stand for darkness and light, respectively.
Reender Kranenborg disagrees, stating that darkness and light have nothing to do with the word guru. He describes this as a folk etymology.
Another etymology of the word “guru” found in the Guru Gita, includes gu as “beyond the qualities” and ru as “devoid of form”, stating that “He who bestows that nature which transcend the qualities is said to be guru”. The meanings of “gu” and “ru” can also be traced to the Sutras indicating concealment and its annulment.
In Western Esotericism and the Science of Religion, Pierre Riffard makes a distinction between “occult” and “scientific” etymologies, citing as an example of the former the etymology of ‘guru’ in which the derivation is presented as gu (“darkness”) and ru (‘to push away’); the latter he exemplifies by “guru” with the meaning of ‘heavy’."