| extreme | |
| faithful(adjective, open, firm, honest, true) | |
| fatal(adjective, dangerous) | |
| fell(adjective, unpleasant, heavy, important, significant) | |
| fervid(adjective, keen, intense, passionate, enthusiastic) | |
| firm(adjective, strong, hard, tough, severe) | |
| formal | |
| full | |
| grim-faced(adjective, dejection) | |
| grim-visaged(adjective, dejection) | |
| gross(adjective, bad, offensive, foul, obvious) | |
| honorable(adjective, honest, straight, grand, upright) | |
| hot(adjective, great, keen, warm, intense) | |
| humorless | |
| long-faced(adjective, somber, dejection, humorless) | |
| majestic(adjective, greatness) | |
| meditative(adjective, set, thoughtful, thinking, stern) | |
| mighty | |
| no joke(expression, adjective, idiom, dullness, weighty) | |
| no laughing matter(expression, adjective, idiom, weighty, aper) |
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| APA | Smart Define. (2016). serious thesaurus. Retrieved September 24, 2026, from https://smartdefine.org/serious/thesaurus/synonyms |
| Chicago | Smart Define. 2016. "serious thesaurus" https://smartdefine.org/serious/thesaurus/synonyms (accessed September 24, 2026). |
| Harvard | Smart Define 2016, serious thesaurus, Smart Define, viewed 24 September, 2026, <https://smartdefine.org/serious/thesaurus/synonyms>. |
| MLA | Smart Define. "serious thesaurus" 24 February 2016. Web. 24 September 2026. <https://smartdefine.org/serious/thesaurus/synonyms> |