impenetrable

adjective

im·​pen·​e·​tra·​ble (ˌ)im-ˈpe-nə-trə-bəl 

1

a

: incapable of being penetrated or pierced

impenetrable armor

b

: inaccessible to knowledge, reason, or sympathy : impervious

an impenetrable heart

2

: incapable of being comprehended : inscrutable

an impenetrable mystery

impenetrably

(ˌ)im-ˈpe-nə-trə-blē 

adverb

Synonyms of impenetrable

• dense
• impervious
• close
• impregnable
• impassable
• impermeable
• thick
• frozen

Examples of impenetrable in a Sentence

The fort's defenses were thought to be impenetrable.

the ancient temple was surrounded by vast stretches of impenetrable jungle

Recent Examples on the Web

Though Best Original Song can seem impenetrable, as the Music Branch that determines the shortlist and nominees for the category has its own insular politics, Swift has inched her way closer to a nomination with every song she’s written for a film. — Marcus Jones, IndieWire, 26 Sep. 2026

Director Padraic McKinley’s debut feature stars Ethan Hawke as a battle-scarred veteran hired to smuggle a fortune in gold across 100 miles of impenetrable wilderness. — Payton Turkeltaub, Variety, 19 Sep. 2026

But even with the Bills having the near-consensus best quarterback in football, this Buffalo team isn't impenetrable, and the Lions have plenty of big-time road experience that could lead to an early-season upset. — Christian Romo, Freep.com, 17 Sep. 2026

For most of our history as a country, and certainly in the post-World War II nuclear era, our borders felt impenetrable. — Michael Silver, New York Times, 11 Sep. 2026

Word History

Etymology

Middle English impenetrabel, from Middle French impenetrable, from Latin impenetrabilis, from in- + penetrabilis penetrable

First Known Use

15th century, in the meaning defined at sense 1a

Rhymes for impenetrable

• penetrable

Kids Definition

impenetrable

adjective

im·​pen·​e·​tra·​ble (ˈ)im-ˈpen-ə-trə-bəl 

1

: impossible to get through or into

impenetrable walls

impenetrable jungle

2

: impossible to understand

an impenetrable mystery

impenetrability

(ˌ)im-ˌpen-ə-trə-ˈbil-ət-ē

noun

impenetrably

(ˈ)im-ˈpen-ə-trə-blē 

adverb