| APA | Smart Define. (2016). tough thesaurus. Retrieved September 29, 2026, from https://smartdefine.org/tough/thesaurus/narrower |
| Chicago | Smart Define. 2016. "tough thesaurus" https://smartdefine.org/tough/thesaurus/narrower (accessed September 29, 2026). |
| Harvard | Smart Define 2016, tough thesaurus, Smart Define, viewed 29 September, 2026, <https://smartdefine.org/tough/thesaurus/narrower>. |
| MLA | Smart Define. "tough thesaurus" 30 March 2016. Web. 29 September 2026. <https://smartdefine.org/tough/thesaurus/narrower> |