Un soodhe idhu Arunachala
Arunachala! Who is smart enough to enter surreptitiously when You, as One Form (ekatma swarupam), pervade every being? It must be your cunning/deceitful plan.
Who can deceive the One who has eyes everywhere and stands as witness in every direction? How can anything or anyone hide the Sun that does not rise or set? This must be your deceitful plan.
OLithu means incandescent/bright, a property of light (OLithu usually means to hide something from someone). Your effulgent form alone will bring everything to light, how then is this possible for others to enter?
It is the mind’s imagination that results in duality such as good and bad, hiding and revealing. These don’t exist in the Pure Being that is Arunachala. So Bhagavan calls him ‘Oruvan’ meaning One (common meaning is a single male person, oruthi is one woman). He refers to the mind’s imagination as deceit. Even then the mind cannot wander without the complicity of Him. Avan indri our anuvum asiyadhu is an old Tamil saying meaning, Without Him not even an atom will move’. So Bhagavan says ‘idhu un soodhe’, that is, this is your deceit/cunning.
(*Note: Soodhu has two meaning, on his deceit, fraud etc, the other is to gamble which often involves deceit, maybe that could be a reason. TRK doesn’t say this, it’s just my additional note).