From Hindupedia, the Hindu Encyclopedia
By M. A. Alwar
Indrāyudham is a neutral form.
It is derived from "indrasyāyudhamiva cāpākṛtitvāt" which means 'Like the weapon of Indra, owing to its shape like the bow'.
It has been denoted in Raghuvaṁśa that,
“He did not allow to remain either the shout of Meghanāda, or his bow, radiant like Indra’s bow, even as autumn brooks not either the roar or the rainbow of a cloud.”
in Manu Smriti
It has been referred to in Manu that
“A wise man, having seen a rainbow in the sky, should not show it to anybody”.
- Shabdakalpadrumah by Raja Radhakantdev, Varadaprasada Vasu, Haricarana Vasu