Synonyms of insoluble

2 Syllables
baffling, tough-nut
3 Syllables
hardline, mystifying, difficult, mysterious, unsolved, obscure, self-willed, obscurer, unpliable
4 Syllables
intractable, alchemistic, unsolvable, unresolved, unconcluded
5 Syllables
unaccountable, unfathomable, inextricable, irresolvable, enigmatical, undefinable, astrological, inexplicable, unexplainable, impenetrable, inexplainable
6 Syllables
indecipherable