| APA | Smart Define. (2015). impenetrable thesaurus. Retrieved September 21, 2026, from https://smartdefine.org/impenetrable/thesaurus/narrower |
| Chicago | Smart Define. 2015. "impenetrable thesaurus" https://smartdefine.org/impenetrable/thesaurus/narrower (accessed September 21, 2026). |
| Harvard | Smart Define 2015, impenetrable thesaurus, Smart Define, viewed 21 September, 2026, <https://smartdefine.org/impenetrable/thesaurus/narrower>. |
| MLA | Smart Define. "impenetrable thesaurus" 10 April 2015. Web. 21 September 2026. <https://smartdefine.org/impenetrable/thesaurus/narrower> |