Difficult or impossible to understand or measure, incomprehensible; unreachable.
In Old English, the noun fathom represented the length of outstretched arms, measured proximately as six feet; it also used to represent physical arms and grasp, and figuratively- power. In Proto Germanic, fathmaz represented an embrace, in Old Norse, fathmr simply represented bosom. In Old Frisian, the word fethem represented a thread, explained by the Oxford English Dictionary as reference to ‘spreading out’.
The Old English verb fathmian referred to an embrace, thus its complementing noun had to be physically tangible. The figurative sense of ‘getting to the bottom of’ or understanding something is dated back to 1620’s.
Why this word?
Unfathomable may describe theories beyond one’s grasp to the same extent it may describe an impossible love. It’s physical connotation to arms and bosom inherently relates this adjective to something we want but cannot have; a profound understanding of something or the object of a feeling.
This word has the power to intensify and farther away urges initially unreachable, burring those six feet underground.
How to use unfathomable?
Do not use this word in vein, make sure that your unreachable is truly so, otherwise you would be misusing this word. The abyssal depths of the ocean are unfathomable to all, Jacques Derrida’s theories are unfathomable to most; his love is unfathomable to me.