The Meaning of the Word Guru
"Guru" is one of those words that has often been misunderstood (now more than ever).
"The word guru means teacher in Sanskrit and other Sanskrit-derived languages like Hindi, Bengali, and Gujarati. It originated in a Hindu context and holds a special place in Hinduism, signifying the sacred place of knowledge (vidya) and the imparter of knowledge. The word comes from the sanskrit root "gru" literally meaning heavy, weighty. Another etymology claimed in Hindu scriptures is that of dispeller of darkness (wherein darkness is seen as avidya, lack of knowledge both spiritual and intellectual), 'gu' meaning darkness, and 'ru' meaning dispeller."
Traditional etymologies elaborating on a word by examining its constituents are in fact quite common. Often, they either involve interpretation of the syllables of the word in connection with verbal roots, or otherwise derive meanings that take syllables as abbreviations of longer terms.
In this particular case, you'll find the etymology at the Upanisads themselves, in the Advaya-taraka-upanisad, not a modern source by any count, listed as one of the 108 classical Upanisads in Mundaka as it is. The verse reads as follows:
गुशब्दस्त्वन्धकारः स्यात् रुशब्दस्तन्निरोधकः।
अन्धकारनिरोधित्वात् गुरुरित्यभिधीयते॥ १६॥
"The word 'gu' is darkness, the word 'ru' is its destroyer; With the destruction of darkness, guru is thus titled."
Verses exploring the word "guru" are also found in the Guru-gita:
गुकाराधकारो ह कारतेज उयते |
अानासकं गुरेव न संशयः ||
"The syllable 'gu' is darkness, and the syllable 'ru' is said to be light; Indeed, there is no doubt that guru is the brahman that swallows ignorance."
"The syllable 'gu' is the disease of this world, the syllable 'ru' is its destroyer; The taker away of the disease of the world, guru is thus defined."
"The syllable 'gu' is that which is beyond qualities, and the syllable 'ru' is that beyond forms; By the abandonment of qualities and forms, guru is thus defined."
"With the first syllable 'gu', he sheds enlightenment over the qualities led by illusion; With the syllable 'ru', he is the great brahman and deliverance from the knots of illusion."
While I am more mystified over the source of the "gu" syllable as "darkness", one of the direct meanings of the syllable "ru", as found in Sir Monier Williams' Sanskrit dictionary, is "to break, shatter". As seen in some of the verses from Guru-gita, the "gu" has been taken as indicating "guNa", the ropes that bind the Atman to mAyA, while the guru is engaged in the "ru", or the act of shattering the illusion. Anything under the guNa is no doubt veiled in avidyA, of which darkness is the mighty emblem , and a common metaphor at that.
Of course, the word also means "heavy", that certainly is one of the dozens of dictionary definitions for the word. It also means "great", "large", and "extended". And it also means "hard to digest", "high in degree", "violent", "vehement", "excessive", "difficult", "hard" and "grievous" even. Yet it also means "important", "serious" and "momentous", "valuable" and even "highly prized". But alas, it also means "haughty" and "proud", even if "venerable" and "respectable". Then it also just means "a spiritual parent or preceptor", or in general "parents and other venerable persons" , these two are as much direct dictionary meanings as any of the others given, even if "heavy" and "weighty" happen to be the first in the list.