Maharshi gave the meaning of Arunāchala:Aruna = Red, bright like fire.
The fire is not ordinary fire which is only hot. This is Jnānāgni (Fire of Wisdom) which is neither hot nor cool.
Achala = a hill.
So it means Hill of Wisdom.
Talks with Sri Ramana Maharshi · Volume I
Talk 216
22nd June, 1936.