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

Altitude is supposed to be impenetrable against tourist teams. —Bennett Durando, Denver Post, 2 Dec. 2025

That said, Stacey is so impenetrable that the women’s anger just doesn’t land effectively. —Shamira Ibrahim, Vulture, 1 Dec. 2025

Ultimately, Dawson suggests that while platforms will have to pull up the drawbridge to under-16s, the castle won’t be completely impenetrable. —Hilary Whiteman, CNN Money, 29 Nov. 2025

Max and Ben ask whether the court is actually leaking more, how newer justices are reshaping its public face, and what Kantor has learned about the culture of secrecy and power inside a long-impenetrable institution. —Ben Smith, semafor.com, 21 Nov. 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