Impenetrable describes something that's impossible to get through. Whether it's a brick wall or a difficult reading passage, something impenetrable won't let you in.

Impenetrable comes from the Latin impenetrabilis, meaning "not to put or get into, enter into." Impenetrable, the adjective, has two meanings. When you're trying to learn a subject that is so complicated, so confusing or so detailed that it seems like it's in another language or from another planet, it's impenetrable — like calculus. Or a physical object can be impenetrable, meaning it is impossible to actually enter or get inside of, like that impenetrable nightclub that has bouncers watching every entrance to keep you and your friends out.

Definitions of impenetrable

adj not admitting of penetration or passage into or through

“an impenetrable fortress”
impenetrable rain forests”
dense, thick
hard to pass through because of dense growth
admitting of penetration or passage into or through

adj permitting little if any light to pass through because of denseness of matter

impenetrable gloom”
dense, heavy
relatively dense in consistency

adj impossible to understand

impenetrable jargon”
incomprehensible, uncomprehensible
difficult to understand

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