The term 'thoroughly immersed' can be described as being ENGROSSED, which is a nine-letter word. ENGROSSED means to be completely absorbed in something, such as a book or activity. It is often used to describe someone who is so focused on something that they are unaware of their surroundings. Being ENGROSSED in something can also mean that one has a deep understanding of the subject matter. Therefore, the term 'thoroughly immersed' can be used to describe someone who is completely absorbed in an activity or subject and has a deep understanding of it.