The term 'deeply ingrained habit' can be summarized by two crossword solutions: 'NATURE' (6 letters) and 'secondnature' (12 letters). NATURE is a term used to describe the physical world, including plants, animals, and other elements of the environment. Secondnature is an expression used to describe something that has become so familiar that it is almost instinctive or automatic. Therefore, a deeply ingrained habit is something that has become so natural that it is almost second NATURE.