If you can't change it, it's immutable. There are many things in life that are immutable; these unchangeable things include death, taxes, and the laws of physics.
The adjective immutable has Latin roots that mean "not changeable." The Latin prefix for not is in, but the spelling changes when the prefix is put before the consonant m. It is im before a root word starting with m as in immutable. If you learn this rule, you'll know the immutable fact that immutable begins with i-m-m.