Answer: origin.
The end of a muscle that is attached to the immovable bone is called the origin.
In some parts I have found that they refer origin of a muscle as the bone, while in other parts I have found that they refer tha origin is part of the muscle.
Anyway, the question is refering to this part: the origin of a muscle, which is a the end of a muscle that is attached an immovable or less movable bone.