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

• dense
• impervious
• impassable
• thick
• impermeable
• close
• 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

This is all meant to be impenetrable to outsiders, which is one reason for the confusion that follows shootings such as the one in Minneapolis. —Charlie Warzel, The Atlantic, 4 Sep. 2025

Yet, across the water, a seemingly impenetrable wall of the invasive plants sways in the wind, several feet tall. —Adriana Pérez, Chicago Tribune, 31 Aug. 2025

At this time, the entirety of the right side of the lunar disk will be directly illuminated by sunlight, presenting the ancient basaltic plains of Mare Serenitatis and Mare Tranquilitatis to exploration, while its left side will remain veiled in impenetrable darkness. —Anthony Wood, Space.com, 29 Aug. 2025

Bottom line Nvidia delivered a strong but not impenetrable quarter. —CNBC, 28 Aug. 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