"Guru" is one of those words that has often been misunderstood (now more than ever). From the Wikipedia page on the Guru:
"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."
However this information may not be completely correct. I have been informed by Pushpa, whose teacher is a very knowledgeable and respected authority in Sanskrit, that
"The etymological claims that 'gu' means darkness and 'ru' means light therefore 'guru' literally means 'the one who brings you from darkness to light' is an idea popularised by frauds and is complete nonsense. Guru is an adjective in Sanskrit meaning heavy. So literally the guru as teacher is 'the heavy guy'."
See also Yoga Journal - 200 Key Sanskrit Yoga Terms:
Guru ("he who is heavy, weighty"): a spiritual teacher
and the Parapsychology Dictionary on Guru at Experience Festival:
Refers to the 'Spiritual Master'. One who has attained perfection in yoga. Also refers to the name for Jupiter as well as the general title for a 'teacher'. Guru literally means 'heavy' and refers to 'heavy with knowledge'. Jupiter is also the largest planet."
There might also (in addition to "heavy") be another definition, which is "difficult". One could interpret this as "the difficult one" or "the one who is difficult to understand". However, "Heavy" is the standard, and correct definition, although because of poor scholarship this is little known in the West.
An alternative point of view is provided by Madhavananda, who advocates a definition closer to that found in Wikipedia (and who wrote to me (email dated 25 May 2007) to reply to an earlier version of this page). He says:
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:
gu-zabdas tv andhakAraH syAt ru-zabdas tan nirodhakaH / andhakAra-nirodhitvAt gurur ity abhidhIyate // 16 //
"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:
gu-kAraz cAndhakAro hi ru-kAras teja ucyate /
ajJAna-grAsakaM brahma gurur eva na saMzayaH // 1.44 //
"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."
gu-kAro bhava-rogaH syAt ru-kAras tan nirodha-kRt /
bhava-roga-haratyAcca gurur ity abhidhIyate // 1.45 //
"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."
gu-kAraz ca guNAtIto rUpAtIto ru-kArakaH /
guNa-rUpa-vihInatvAt gurur ity abhidhIyate // 1.46 //
"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."
gu-kAraH prathamo varNo mAyAdi-guNa-bhAsakaH /
ru-kAro'sti paraM brahma mAyA-bhrAnti-vimocanam // 1.47 //
"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.
See also What does the term 'Guru' mean? , for a number of definitions
Now, I in no way claim to be any sort of authority in this subject. All I can do therefore is present these contending points of view, and let people decide what definition or definitrions they prefer.