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

But the Broncos aren’t just about putting the ball in the back of the net — their defense has allowed just six goals all season and is led by near-impenetrable sophomore goalkeeper Ava de Leest. — Shaun Goodwin, Idaho Statesman, 17 Oct. 2026

Even top experts have found OpenAI’s proof of blowup to be completely impenetrable. — Joseph Howlett, Scientific American, 13 Oct. 2026

Cue an enthralling chase sequence, doubling back through those higgledy-piggledy Portland streetscapes, toward the woods — the impenetrable thickets and thorns of which magically part in her presence. — Guy Lodge, Variety, 11 Oct. 2026

Medical bills are often impenetrable documents littered with complex codes and terminology, and sorting through charges and details about insurance coverage can feel intimidating, if not downright paralyzing. — Jennifer Schlinger, The Conversation, 6 Oct. 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