Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unfathomable; he lists it under infinite, mysterious.
He lists it without comment; the entries above explain its neighbors.
Synonyms
infinitemysteriousabsoluteabstruseboundlesscabalisticcountlessdarkenigmaticaleternalhiddenillimitableimmeasurableincomprehensibleinexplicableinnumerableinscrutableinterminablelimitlessmeasurelessmysticmysticalnumberlessobscureoccultreconditesecrettranscendentalunboundedunconditionedunfathomedunknownunlimitedunmeasured