Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.2.1

1.2.1

naiveha kiṃcanāgra āsīnmṛtyunaivedamāvṛtamāsīt | aśanāyayāśanāyā hi mṛtyustanmano'kurutātmanvī syāmiti | so'rcannacarattasyārcata āpo'jāyantārcate vai me kamabhūditi tadevārkasyārkatvaṃ kam̐ ha vā asmai bhavati ya evametadarkasyārkatvaṃ veda

In the beginning there was nothing (to be perceived) here whatsoever. By Death indeed all this was concealed,--by hunger; for death is hunger. Death (the first being) thought, 'Let me have a body.' Then he moved about, worshipping. From him thus worshipping water was produced. And he said: 'Verily, there appeared to me, while I worshipped (arkate), water (ka).' This is why water is called ar-ka. Surely there is water (or pleasure) for him who thus knows the reason why water is called arka.