In a way that cannot be measured, understood, or reached; impossibly deep or mysterious.
From 'fathomless' (late 1600s) + '-ly' adverb suffix. 'Fathom' comes from Old English 'fæthm' meaning an arm's span or measure of depth. The '-less' suffix (without) was added to mean 'immeasurable.'
Ancient sailors used their own arm spans to measure ocean depth—a 'fathom' was literally how far they could stretch! When something became 'fathomless,' it meant the ocean was so deep their measurement system broke down, which is why the word now means any mystery too vast to understand.
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