Impossible to understand or measure; too deep or complex to comprehend fully.
From 'un-' (not) + 'fathom' + '-able.' 'Fathom' comes from Old English 'fæðm' (embrace, span of arms), used for measuring water depth. The negative prefix makes it 'impossible to measure.'
A fathom is the length of an outstretched person's arms—originally how sailors measured water depth by dropping a weighted rope—so 'unfathomable' literally means 'too deep to measure with your body-length'!
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