127 entries under tough topic. ×

50
severe(adjective, calm, help, bad, strong)
43
weighty(adjective, good, dull, hard, great)
32
critical(adjective, hard, great, severe)
30
sober(adjective, calm, dull, quiet, cool)
30
difficult(adjective, hard, easy, harsh)
29
grievous(adjective, help, hard, great)
26
hard(adjective, strong, easy, severe)
25
arduous(adjective, hard, easy, difficult)
20
grim(adjective, dull, strong, hard)
20
stubborn(adjective, strong, hard, firm)
19
cruel(adjective, strong, hard, severe)
19
hard-nosed(adjective, strong, hard, severe)
19
heavy(adjective, strong, hard, great)
19
rough(adjective, bad, strong, hard)
19
rugged(adjective, strong, hard, difficult)
16
pressing(adjective, hard, difficult, big)
14
urgent(adjective, hard, severe, difficult)
8
challenging(adjective, hard, difficult, importance)
7
austere(adjective, set, strong, hard)
5
calm(adjective, cool, sensible, relaxed)
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APASmart Define. (2016). serious thesaurus. Retrieved September 28, 2026, from https://smartdefine.org/serious/thesaurus/synonyms
ChicagoSmart Define. 2016. "serious thesaurus" https://smartdefine.org/serious/thesaurus/synonyms (accessed September 28, 2026).
HarvardSmart Define 2016, serious thesaurus, Smart Define, viewed 28 September, 2026, <https://smartdefine.org/serious/thesaurus/synonyms>.
MLASmart Define. "serious thesaurus" 24 February 2016. Web. 28 September 2026. <https://smartdefine.org/serious/thesaurus/synonyms>