Who is the Brahmana changed into a woman in the Rig Veda?
The Brahmana who became a woman is Indra himself! I found the answer to my question in this excerpt from the Brihaddevata, an ancient work by the sage Shaunaka, which describes the different gods that Rig-Vedic verses are addressed to and the stories of how those verses were heard from the gods. Here's what Shaunaka says about verse 19:
In 'Downward' (adhaḥ: viii.33.19) a girl addressed (who appeared) with the characteristics of a woman; for the chastiser of Paka (Indra) made love to that Danava maiden, the elder sister of Vyamsa, by reason of his (Indra's) youthful desire (yuva-kāmyā).
So it looks like Indra disguised himself as a woman in order to have an affair with the sister of the Danava Vyamsa. So in verse 19, Vyamsa's sister is telling Indra to keep his eyes down so that Vyamsa doesn't spot him.
Indra subsequently killed the demon Vyamsa, as described in this hymn of the Rig Veda:
Thou art mine own, O Maghavan, whom Vyaṁsa struck to the ground and smote thy jaws in pieces. But, smitten through, the mastery thou wonnest, and with thy bolt the Dāsa's head thou crushedst.
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