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

Right now, though, Payton is betting he’s built a fortress impenetrable to such thoughts as long as there is still an opponent ahead. —Parker Gabriel, Denver Post, 24 Jan. 2026

BitLocker’s encryption algorithms have proven impenetrable to prior law enforcement attempts to break in, according to a Forbes review of historical cases. —Thomas Brewster, Forbes.com, 23 Jan. 2026

All night, the fog had stubbornly hidden the stars in the dark, impenetrable sky, but now the sliver of light widened little by little into a luminous band, a hazy phosphorescence that slowly spread and took over. —Literary Hub, 22 Jan. 2026

Powerful and charismatic, your presence is especially magnetic at this time, making your energy impenetrable and impossible to ignore. —Valerie Mesa, PEOPLE, 19 Jan. 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