impenetrable
adj.adj. impossible to pass through or see through. It can also describe a piece of writing or an idea that is too difficult to understand.
adj. impossible to pass through, enter, or see through; metaphorically describes text or concepts that are impossible to comprehend or interpret.
The thick forest was impenetrable even with a sharp tool.
The lawyer's argument was so full of technical jargon that it remained impenetrable to the average juror.
The fortress was protected by impenetrable stone walls and a series of complex moats that had discouraged invaders for centuries.
From Middle English inpenetrabel, inpenetrabyle, from Middle French impenetrable or directly from its etymon, Latin impenetrābilis. By surface analysis, im- + penetrable.
Often used both literally for physical barriers and figuratively for intellectual difficulty.