impenetrable

adjective

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

1

a

: incapable of being penetrated or pierced

b

: inaccessible to knowledge, reason, or sympathy : impervious

2

: incapable of being comprehended : inscrutable

impenetrably

(ˌ)im-ˈpe-nə-trə-blē 

adverb

Synonyms

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

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

And in the chaotic months since, clues about Tice’s fate have begun to seep through the walls of Syria’s once-impenetrable prisons and palaces. —Rob Picheta, CNN Money, 28 Oct. 2025

Yes, only two additional albums in 30 years since his 1995 debut, with a 14-year gap between the second and third, but an impenetrable recipe for everyone from Beyoncé to Kendrick Lamar. —Kevin Powell, MSNBC Newsweek, 16 Oct. 2025

For instance, in The Trial, Kafka looks at alienation vis-à-vis a faceless, impenetrable bureaucracy. —Big Think, 14 Oct. 2025

For every National Business Solutions and Darmine Devices that appear impenetrable in their genericness, there’s a small-time local establishment finding a way to sell a mediocre product to Robinson. —Christian Zilko, IndieWire, 13 Oct. 2025

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