The word "unpenetrable" means something that cannot be penetrated or entered. There are many synonyms for this word, such as impervious, impenetrable, invulnerable, unconquerable, untouchable, and unassailable. Impervious refers to something that is unaffected by external forces, while impenetrable refers to something that cannot be penetrated physically or mentally. Invulnerable means something that is incapable of being harmed or injured. Unconquerable refers to something that cannot be defeated, while untouchable describes something that is out of reach or beyond influence. Finally, unassailable refers to something that is beyond attack or criticism. All of these synonyms can be used interchangeably with the word "unpenetrable" to convey similar meanings.