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

Chalk it up to the leftover hubris of late-20th-century American empire and the grand illusion of a liberated civilization thriving behind an impenetrable fortress. — Andy Andersen, Vulture, 16 Aug. 2026

The rule also failed to tackle all of the ways trusts and other vehicles provide impenetrable secrecy for the wealthy and the crooked alike. — Casey Michel, The Atlantic, 14 Aug. 2026

Cody erects impenetrable emotional walls around him that deny him from asking for help and creating community. — Randy Myers, Mercury News, 12 Aug. 2026

The pair will share the screen with Will Smith and AnnaSophia Robb, who play two FBI agents investigating a seemingly impossible murder within the impenetrable walls of a maximum-security prison. — Matt Grobar, Deadline, 7 Aug. 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