Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The millstone in a dream indicates a scholar or a ruler. It may also indicate a husband and wife. Its turning indicates travel and sustenance.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
The millstone indicates death, war, or hardship. Whoever sees himself grinding with it will fall into hardship.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The millstone is interpreted as sustenance. The flour from it is lawful money.