Without letters/schooling; unlearned in the formal rabbinic or scribal education
Agrammatos (from a- privative + gramma, 'letter') means 'without letters' — unlearned, illiterate, or untrained in formal schooling. In Acts 4:13, it describes Peter and John as 'unschooled' (agrammatos) in the eyes of the Jewish leaders, who were astonished that such men could speak with such authority. The word does not necessarily mean unable to read, but rather without formal rabbinic or scribal training.
Acts 4:13 is one of the most theologically charged uses of agrammatos in the New Testament. The Sanhedrin — the supreme religious court — marveled at Peter and John's boldness and recognized that they had 'been with Jesus.' The very men who lacked academic credentials confounded the academically credentialed. This fulfills the principle of 1 Corinthians 1:27: 'God chose the foolish things of the world to shame the wise.' The Holy Spirit's power is not dependent on human education. This does not devalue learning, but it does relativize it — the Spirit can work through any vessel He chooses.