Impenetrable means impossible to get through or to understand. Impenetrable forest, impenetrable prose, an impenetrable wall.
The physical sense covers armour, jungle and fog. The figurative sense is much commoner and is a real criticism of writing: impenetrable prose is prose that defeats a willing reader, and the fault is the writer's. It also describes expressions — an impenetrable face gives nothing away. Note impenetrability; and penetrate, which does the opposite work.
1adjectiveimpossibleImpossible to get through.
“The jungle was an impenetrable wall of vines and thorns.”
2adjectiveimpenetrableImpossible to understand.
“The instructions were so full of jargon that they were impenetrable.”
impenetrable is a quality of being closed off.
Latin impenetrabilis is in- plus penetrabilis, from penetrare, to pass into. Penetrate and penetration come from the same passing in.
Proto-Indo-European
*pen-
Latin
impenetrābilis
Middle French
impenetrable
Middle English
inpenetrabel
English, today
impenetrable
From Middle English inpenetrabel, inpenetrabyle, from Middle French impenetrable or directly from its etymon, Latin impenetrābilis. by surface analysis, im- + penetrable.
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire