50 entries under hard topic. ×

25
fixed(adjective, secure, rough, firm)
25
unalterable(adjective, calm, strong, secure)
12
firm(adjective, calm, strong, secure)
10
inflexible(adjective, set, secure, rough)
8
resolute(adjective, calm, strong, determined)
8
rigid(adjective, set, regular, fixed)
6
immovable(adjective, set, steady, regular)
6
obstinate(adjective, set, strong, severe)
5
adamant(adjective, firm, fixed, inflexible)
4
determined(adjective, firm, resolute, rigid)
4
dogged(adjective, set, strong, firm)
4
intractable(adjective, set, severe, rough)
4
rigorous(adjective, resolute, rigid, inflexible)
4
stiff(adjective, set, severe, rough)
4
unbending(adjective, firm, rigid, inflexible)
4
uncompromising(adjective, secure, rough, firm)
4
unyielding(adjective, firm, steady, fixed)
3
iron(adjective, set, stiff, rigid)
3
single-minded(adjective, set, stiff, rigid)
3
steely(adjective, set, stiff, rigid)
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APASmart Define. (2015). immutable thesaurus. Retrieved September 22, 2026, from https://smartdefine.org/immutable/thesaurus/synonyms
ChicagoSmart Define. 2015. "immutable thesaurus" https://smartdefine.org/immutable/thesaurus/synonyms (accessed September 22, 2026).
HarvardSmart Define 2015, immutable thesaurus, Smart Define, viewed 22 September, 2026, <https://smartdefine.org/immutable/thesaurus/synonyms>.
MLASmart Define. "immutable thesaurus" 10 April 2015. Web. 22 September 2026. <https://smartdefine.org/immutable/thesaurus/synonyms>