12 letters · 18 points in Scrabble
adjective not admitting of penetration or passage into or through
adjective permitting little if any light to pass through because of denseness of matter
adjective impossible to understand
originFrom 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 3.0