Sivaite, Buddhist or Ghostistic?
The first etymology that I heard was, the guardian (nAtha) of ice (maMju) mountains (Himalayalas, or Mount Kailasa) or Siva. The name became popular because of a famous temple in my region. Until very recent I would explain this etymology to people. It must be noted here that though the name sounds so much north Indian only in my region Siva is called Manjunatha. I found it bit strange. But as my understanding grew I found this name in fact has a Buddhist beginning.
The first Manjunatha temple was built at a place called Kadri in Mangalore. However, as I have already discussed before this was in fact a Buddhist monastery which was converted to Saiva temple. This event deals with assimilation of Tantrik Buddhism with Tantrik Sivaism. One of the Buddhist deities in that monastery was Manjusri. In my opinion, since Manjusri is an important Bodhisattva for Tantrik Buddhism the name Manjunatha might have been conceived from Manjusri. This I felt is the most plausible scenario. This was until the blogger Ravi Mundkur brought one more angle to the name.
In his series of posts on Kadri and Natha cult he proposes the native beginning for this name from ghostistic tradition. Ravi has developed his view based on legends surrounding Natha cult figure Matsyendranatha. Manju was supposedly one of his sons and a spirit worship was inititated after his death. This spirit Manju became deity Manju Natha in Saiva tradition. I am not yet convinced of this angle as Buddhist deities getting Saiva make over has been observed in the history. And legends can not be authentic accounts.
From Wikipedia on Avalokitesvara;
according to recent research, the original form was Avalokitasvara with the ending svara ("sound, noise"), which means "sound perceiver", literally "he who has perceived sound" (i.e. perceiver of the cries of the sentient beings who need his help). This is the exact equivalent of the Chinese translation Guan Yin. This name was later supplanted by the form containing the ending -īśvara, which does not occur in Sanskrit before the seventh century. The original form Avalokitasvara already appears in Sanskrit fragments of the fifth century.
The original meaning of the name fits the Buddhist understanding of the role of a bodhisattva. The reinterpretation presenting him as an īśvara shows a strong influence of Shaivism, as the term īśvara was usually connected to the Hindu notion of a creator god and ruler of the world. Attributes of such a god were transmitted to the bodhisattva
When Avalokitasvara can become Avalokitesvara, Manjusri turning into Manjunatha isn't all that strange. But what came out of Ravi's posts is my ignorance about the word 'manju'.
Until Ravi mentioned about a Sanskrit word "manju" meaning beautiful, charming, pleasant etc..., I believed the word manju in Manjunatha is in fact a Sanskrit word meaning ice, coldness, dew, fog etc...! It turns out that manju that I know is in fact a Dravidian word. I didn't know. Had I known before I would have grown into Indic manju than Dravidian manju.
Therefore, the common derivation of Manjunatha for Siva is a mix of Dravidian word manju with Indic word natha. However, ice formations are not observed in South India therefore 'dew' manju might have been stretched to incorporate 'ice' in recent times.
May be Buddhist Manjusri and Kannada word manju together might have given the idea of Manjunatha.