the word "unfathomably" is built upon the old english noun "fathom," which originally denoted the span of one's outstretched arms, a unit for measuring depth, especially of water. the prefix "un-" negates this, creating "unfathomable," meaning "incapable of being measured or understood." the adverbial suffix "-ably" is then added, derived from latin -abilis via old french, to form the adverb describing an action or quality performed to an immeasurable degree. thus, the term's journey from a concrete physical measurement to an abstract intensifier for the incomprehensible illustrates a classic path of semantic broadening and grammaticalization.

Structure: un (not) + fathom (measure) + ably (in a manner of) - un: a prefix of old english origin, meaning "not," used to form negatives. - fathom: from old english fæthm, meaning "the length of the outstretched arms," thus a measure of depth or extent. - ably: an adverbial suffix of latin origin (via old french), meaning "in a manner that is capable of" or "to a degree that is."