So unclear or confusing that it's impossible to understand what is being said or meant.
From Latin 'intelligibilis' (able to be understood), with the prefix 'un-' added to negate it. The root comes from 'intelligere' (to understand), combining 'inter-' (between) and 'legere' (to choose or read).
The root 'intelligere' literally means 'to read between'—so when something is 'unintelligible,' you literally cannot read between the lines because the lines themselves are a blur. This is why AI sometimes produces 'unintelligible' output—it's literally missing the meaning between the words.
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