1
general::
adj. impossible to enter/get through VERBS appear, be, look, seem | become | find sth They found the jungle virtually impenetrable. ADV. almost, nearly, virtually 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.
Oxford Collocations Dictionary