Prior the Thaipusam festival this year (2012) I had a doubt on what is 'Harohara'. Stemming from Hinduism, it definitely CAN'T be meaningless !!
We seem to hear it quite a bit in Murugan/Skanda/Karthigeya worship.. So yea, I went to search about it and here I am to share what I found with you..The word 'arOharA' is a shortened form of the phrase 'ara harO harA'.
Its meaning is:
"Oh God Almighty, please remove our sufferings and grant us happiness".
This was first used by Saivites (followers of Saivism - Devotees who regard Shiva as the Supreme), at the time of Saint Thirunyanasambandhar. During Thirunyanasambandhar's journeys, the bearers of his 'pallaakku' and followers had the habit of chanting (meaningless) phrases - like 'yElEloh yElEloh', as a form of stress relief. Thirunyanasambandhar taught them to replace these with the meaningful 'ara harO harA'.
Over time, the use of 'ara harO harA' faded among the 'Saivites' but lived on with the 'Kaumaras' (Devotees who regard Murugan as the Supreme). It was further shortened to the present'arOharA'.
Whenever the devotee says 'vetrivEL Muruganukku arOharA' he/she is sending a humble message to God Almighty - Lord Murugan - the Bearer of the 'VetriVEl' (Victorious spear) - to grant refuge from all unpleasantness in life.
Explanation extracted from http://www.kaumaram.com/articles/arohara.html