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

Miami has rekindled memories of its 2001 national championship team behind a defense that went from porous to nearly impenetrable in its first season under coordinator Corey Hetherman. —Fiifi Frimpong, New York Daily News, 9 Jan. 2026

Miami has rekindled memories of its 2001 national championship team behind a defense that went from porous to nearly impenetrable in its first season under coordinator Corey Hetherman. —Dallas Morning News, 8 Jan. 2026

Yes, some of the attacking intent was stripped from recent games, but that was necessary in bolstering their impenetrable deep block at the home of the defending champions. —Beren Cross, New York Times, 2 Jan. 2026

Look to the upper region of the lunar crescent to find the menacing form of the Aristotles crater with its eastern rim bathed in impenetrable shadow. —Anthony Wood, Space.com, 26 Dec. 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