'arOharA' is a shortened form of the phrase 'ara harO harA'. Its meaning is:
Oh God Almighty, please remove our sufferings and grant us salvation.
This was first used by Saivaites (followers of Saivam - worship of Shiva), at the time of Thirunjanasambandhar. During Thirunjanasambandhar's journeys, the bearers of his 'pallakku' and followers had the habit of chanting (meaningless) phrases - like 'yElEloh yElEloh', as a form of stress relief. Thirunjanasambandhar taught them to replace these with the meaningful 'ara harO harA'. Over time, the use of 'ara harO harA' faded among the 'Saivaites' but lived on with the 'Kaumaras' (Murugan Devotees). 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.
We, who regard Lord Murugan as The Supreme One, can vigorously say:
'vetrivEl iRaivanukku arOharA'
Sri Eswaran