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

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

Surrounded by highly skilled bodyguards, Safra had created a haven in Monaco in the form of an impenetrable penthouse apartment, equipped with state-of-the-art security systems. —Alex Gurley, PEOPLE, 21 Dec. 2025

Tech leaders’ business success and wealth, inflated by years of near-zero interest rates, fed an impenetrable faith in their own exceptionalism. —Foreign Affairs, 16 Dec. 2025

Shopping deals and savory recipes were interspersed between updates on the growing second World War and analysis about the United States’ impenetrable defensibility. —Angele Latham, Nashville Tennessean, 7 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