1 impossible to enter/get through
VERBS appear, be, look, seem | become | find sth They found the jungle virtually impenetrable.
ADV. almost, nearly, virtually
2 impossible to understand
VERBS be, seem | become | remain | find sth
ADV. almost, virtually | somewhat I find his style somewhat impenetrable. | seemingly
PREP. to The language of this document would be impenetrable to anyone except a specialist.