the word 'unfathomable' is built upon the old english noun 'fæðm', which originally meant the embracing arms or the distance between the outstretched arms, used as a unit of measurement for depth. the verb 'to fathom' evolved from this, meaning to measure depth by embracing or encircling with the arms, and later, to understand something deeply. the negative prefix 'un-' and the adjective-forming suffix '-able' were added, creating a term that literally means "not capable of being measured to its bottom." this concrete sense of immeasurable depth evolved metaphorically to describe ideas, emotions, or situations that are too profound or complex to be fully comprehended.

Structure: un (not) + fathom (measure) + able (capable of) un- (old english, a prefix meaning "not"); fathom (old english, a noun/verb meaning a measure of length or to measure depth); -able (latin via old french, a suffix forming adjectives meaning "capable of, fit for").