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

• impassable
• impassible
• impermeable
• impervious
• 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

Most of those minerals are currently buried beneath thick sheets of previously impenetrable ice. —Mack Degeurin, Popular Science, 10 Apr. 2025

Image Border The Wall The most memorable promise that Mr. Trump made on the campaign trail was to erect an impenetrable barrier along the entire southern border, which is almost 2,000 miles, and to have Mexico pay for it. —Miriam Jordan, New York Times, 18 Jan. 2025

Even more impressively, against a Colorado defense that was impenetrable Saturday, with the Rays lacing the balls all over the field and the Rockies making a reel of highlight plays. —Marc Topkin, The Orlando Sentinel, 30 Mar. 2025

The difficult terrain, impenetrable vegetation, and challenging navigation mean most hikers cover at least 50 percent more distance than that. —Alex Hutchinson, Outside Online, 25 Mar. 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