What is the word comes from title of a book written by an Arabian mathematician around ad 800?

You are likely referring to the book "The Book of Algebra" written by the Persian mathematician Muhammad ibn Musa al-Khwarizmi around 820 AD.

The word "algebra" itself comes directly from the title of this book. The Arabic title was "al-Kitāb al-mukhtaṣar fī ḥisāb al-jabr wa'l-muqābala" which translates to "The Compendious Book on Calculation by Completion and Balancing".

The word "al-jabr" in the title, which means "completion" or "restoration", became "algebra" through a series of translations and transformations over centuries.