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

David Benioff wrote 25th Hour (great), Troy (good), and then Stay, this nearly impenetrable psychological thriller that plays out like The Machinist through a David Lynch filter. —Roxana Hadadi, Vulture, 20 Mar. 2026

To regulators, crypto appeared as one impenetrable blur of internet scam. —Clara Molot, Vanity Fair, 17 Mar. 2026

Quickly, everyone form an impenetrable mass in front of the four feet of bar while the solitary bartender struggles to hear anyone’s order! —Literary Hub, 13 Mar. 2026

The material was porous and crumbly, allaying fears that the fuel might’ve fused into a diamond-hard mass impenetrable even to drills. —Yusuke Maekawa, Bloomberg, 8 Mar. 2026

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