In a way that is impossible or extremely difficult to picture or conceive in your mind.
From English un- (not) + imaginable (able to be imagined) + -ly (adverb suffix). The root 'imagine' comes from Latin imaginari, meaning 'to form an image of.' The prefix 'un-' negates the concept, making it mean the opposite of imaginable.
This word perfectly captures how human brains struggle with extreme numbers—we can imagine 10 apples or even 1,000 apples, but unimaginably large numbers like a trillion break our mental picture-making abilities completely.
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