adjective

impénétrable

/ɛ̃.pe.ne.tʁabl/

adjective/ɛ̃.pe.ne.tʁabl/

impenetrable; not understandable, mysterious

“une barrière impénétrable” — an impenetrable barrier

Borrowed from Latin impenētrābilis. By surface analysis, im- + pénétrable.