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

In this archetype, the leader is a stoic force—strong, steady, and impenetrable. —Julian Hayes Ii, Forbes, 24 Mar. 2025

His manifesto, dismissed in the 1990s as impenetrable, is now the subject of YouTube videos drawing millions of views apiece. —Charles Homans, New York Times, 22 Mar. 2025

But the impenetrable bureaucracy that had sprung up at USAGM has long counteracted their work. —The Editors, National Review, 20 Mar. 2025

Samples with microbes ended up in one chamber, the L-serine in the other, and the barrier, formed using a gel, was formulated to be permeable to microbes but impenetrable by abiotic particles. —Ars Technica, 12 Feb. 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