Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Brain (or marrow) in a dream is buried wealth and hidden knowledge. Eating it indicates gaining stored wealth or hidden knowledge.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
The brain indicates the mind and prudence. Whoever sees that he has no brain is an ignorant, foolish man.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The brain is interpreted as wealth. Whoever sees himself taking out a person's brain is seizing his wealth.