40 entries under deep topic. ×

20
impervious(adjective, set, hard, tough, solid)
19
compact(adjective, close, set, hard, tough)
16
dense(adjective, set, hard, tough, firm)
15
solid(adjective, close, set, hard, tough)
14
opaque(adjective, set, solid, heavy)
13
firm(adjective, close, set, hard, solid)
10
heavy(adjective, set, hard, tough, solid)
9
close(adjective, set, hard, tough, solid)
7
condensed(adjective, set, hard, tough, solid)
6
abstruse(adjective, obscure, difficult, strange)
6
compressed(adjective, set, hard, tough, firm)
5
baffling(adjective, obscure, solid, mysterious)
4
concrete(adjective, set, hard, tough, solid)
4
esoteric(adjective, obscure, difficult, strange)
4
indecipherable(adjective, obscure, mysterious, incoherent)
4
uncomprehensible(adjective, knowledge, obscure, dark, superior)
3
crowded(adjective, close, set, solid, heavy)
2
concentrated(adjective, set, hard, tough, firm)
2
consolidated(adjective, close, set, firm, solid)
2
solidified(adjective, set, hard, tough, solid)
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APASmart Define. (2015). impenetrable thesaurus. Retrieved September 22, 2026, from https://smartdefine.org/impenetrable/thesaurus/synonyms
ChicagoSmart Define. 2015. "impenetrable thesaurus" https://smartdefine.org/impenetrable/thesaurus/synonyms (accessed September 22, 2026).
HarvardSmart Define 2015, impenetrable thesaurus, Smart Define, viewed 22 September, 2026, <https://smartdefine.org/impenetrable/thesaurus/synonyms>.
MLASmart Define. "impenetrable thesaurus" 10 April 2015. Web. 22 September 2026. <https://smartdefine.org/impenetrable/thesaurus/synonyms>