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

The Thunder’s perimeter defense sometimes feels impenetrable and is anchored on the back line by Chet Holmgren, one of the best rim protectors in basketball. —James L. Edwards Iii, New York Times, 27 May 2026

Achilles grew up to be the greatest warrior of his age, his armor impenetrable, his enemies routed. —Nick Lichtenberg, Fortune, 26 May 2026

Tall, impenetrable, like the Whitfields themselves. —ABC News, 26 May 2026

The building of the case is well done but what’s most memorable is the impenetrable brick wall of denial constructed by Shirilla’s family and friends in the face of that evidence. —Stuart Miller, Los Angeles Times, 18 May 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