If you say that someone is in a rut, you disapprove of the fact that they have become fixed in their way of thinking and doing things, and find it difficult to change. You can also say that someone's life or career is in a rut.

A rut is a deep, narrow mark made in the ground by the wheels of a vehicle.

The rut is the period of the year when some animals such as deer are sexually active, and the males fight each other to mate with the females.