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

• impassable
• impassible
• impermeable
• impervious
• 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

Built inside a mountain some 300 feet underground and reinforced by layers of concrete, the plant, which is the most likely target of a potential American strike, is impenetrable by any bomb except the GBU-57 Massive Ordnance Penetrator (MOP). —Adeola Adeosun, MSNBC Newsweek, 21 June 2025

This must be key to Trump’s impenetrable calculations. —Nick Paton Walsh, CNN Money, 19 June 2025

While no cyber defense is impenetrable, fixating on technical design proves futile when incentives, incompetence and indifference undermine internal controls design, implementation and effectiveness. —Noah Barsky, Forbes.com, 11 June 2025

And where leadership believes their infrastructure is impenetrable, red teams often walk through the front door with nothing more than confidence and a cover story. —Jochen Schwenk, Forbes.com, 4 June 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