Inaccessible (adjective)

1. Not able to be reached, entered, or used; not available.
2. Not able to be approached or contacted; not able to be communicated with.
3. Not able to be understood or comprehended; not able to be grasped.

Origin:

From the latin word "inaccessibilis" meaning "not able to be reached".

Examples:

1. The island was inaccessible due to the rough seas.
2. The document was stored in an inaccessible location.
3. The building was inaccessible to people with disabilities.
4. The information was inaccessible due to a technical error.
5. The theory was considered inaccessible to most people due to its complexity.