أُمِّي – أُمِّيُّون
The word ʾummī occurs twice whilst the plural ʾummiyyūn occurs four times.
In his explanation of Sūrah al-Jumuʿah (p. 1016), al-Saʿdī explains the word ʾummiyyīn to mean those from amongst the Arabs and others besides them who are not from the People of the Book, who do not have a revealed scripture and no trace of a divine message.
Badawi and Abdel Haleem (p. 75) provide two definitions. Firstly, it means unlettered, unable to read as in (7:158) when Allāh said:
فَآمِنوا بِاللَّهِ وَرَسولِهِ النَّبِيِّ الأُمِّيِّ
{So believe in Allāh and His Messenger, the unlettered Prophet}
The second signification, is similar to what al-Saʿdī states, in that it refers to a pagan, a person with no revealed scripture as in (3:20) when Allāh says:
وَقُل لِلَّذينَ أوتُوا الكِتابَ وَالأُمِّيّينَ أَأَسلَمتُم
{…ask those who were given the Book, as well as those without one, ‘Do you surrender yourselves entirely to Him too?’}
Bibliography
Badawi, Eslaid M., and Muhammad Abdel Haleem. Arabic – English Dictionary of Qurʾānic Usage. Leiden: Brill, 2008.
al-Saʿdī, ʿAbd al-Raḥmān b. Nāṣir. Taysīr al-Karīm al-Raḥmān fī Tafsīr Kalām al-Mannān, ed. Dr. ʿAbd al-Raḥmān b. Muʿallā al-Luwayḥiq. 2nd Edition. al-Riyāḍ: Dār al-Salām, 1422/2002.