'densely-packed' is a compound formed from the adverb 'densely' and the past participle 'packed'. 'densely' comes from 'dense' + adverbial suffix '-ly', and 'packed' is the past participle of 'pack'.

'dense' originates from Latin 'densus' (meaning 'thick, crowded'), passed into Middle French and then Middle English as 'dense'. 'pack' developed via Middle English (e.g., 'pakken'/'packen') from Germanic roots (compare Middle Dutch 'pakken'), and the past participle 'packed' became established in Modern English. The compound structure 'densely-packed' is a modern English adjectival phrase formed by combining these elements.

Originally Latin 'densus' meant 'thick' or 'crowded'; over time 'dense' came to mean 'closely compacted' and 'packed' meant 'filled tightly'. Together the compound retained the combined sense of 'tightly filled or crowded'.