What's unalienable cannot be taken away or denied. Its most famous use is in the Declaration of Independence, which says people have unalienable rights of life, liberty, and the pursuit of happiness.
To find the origins of the word unalienable, we can look at the root, alien, which comes from the Latin alienus, meaning "of or belonging to another." This provides the basis for our word, with the prefix un- providing the turnaround "not," and the suffix -able providing the idea of capability. Therefore, we get “not able to be denied.” Oh, and if you are wondering about the common argument as to whether it is "unalienable" or "inalienable," either is correct.