I first learned of this from my Verso Radical Diary, although I’m not sure how they arrived at the precise date. Her essay was published in 1855 in an Ahmednagar journal, Dnyanodaya which, in addition to “disseminating information” on emerging scientific fields, also published pieces on morality and religion. The following is a snippet from “About the Griefs of the Mangs and the Mahars:”
[….] “Now obviously, if the Vedas are only for the brahmins, they are absolutely not for us. Teach us, O Lord, thy true religion so that we can all lead our lives according to it. Let that religion, where only one person is privileged and the rest are deprived, perish from the earth and let it never enter our minds to be proud of such a religion. [….] O learned pandits, wind up the selfish prattle of your hollow wisdom and listen to what I have to say.”[….] — translated by Maya Pandit
Muktabai (1841 -?) (not to be confused with this earlier Muktabai, a saint in the Varkari tradition) was only fourteen years of age when she wrote an “About the Griefs of the Mangs and the Mahars.” This is said to be the earliest surviving piece of writing by an “untouchable” [the preferred term today is ‘Dalit’] woman. We have precious little biographical information about her, although we know she was taught by Savitrabai Phule (1831-1897), a famous Indian social worker, reformer and poet.
Further reading: The Varna and Caste System in India: A Basic Bibliography, and B.R. Ambedkar: A Basic Bibliography.