In a manner that is impossible to count; without limit or number; infinitely or endlessly.
From countless (not able to be counted) + -ly (adverb suffix). Countless derives from count + -less (without), with the adverb form emerging to describe manner of action or existence.
When poets describe stars as shining 'countlessly' in the sky, they're capturing something real about human perception—there are so many that our brains literally cannot count them, which is why the word feels more honest than just saying 'many.'
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