12 letters · 22 points in Scrabble
adjective of depth; not capable of being sounded or measured
adjective resembling an abyss in depth; so deep as to be unmeasurable
adjective impossible to come to understand
originFrom un- (prefix meaning ‘not’) + fathom (“to measure the depth of (water); (figurative) to deeply understand (someone or something)”) + -able (suffix forming adjectives denoting things not able or fit to be done).
Etymology from Wiktionary, CC BY-SA 3.0