In a thick or closely packed way, with many things crowded together in a small space. The forest was densely packed with trees.
From the adjective 'dense' (from Latin densus meaning thick or closely woven) plus the adverb suffix -ly. The Latin root relates to the idea of compactness and closely-set elements.
When we describe cities as 'densely populated,' we're using a word that originally described how tightly woven fabric was—the Romans used densus for cloth! It's a perfect example of how practical physical descriptions get borrowed for abstract situations.
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