The phrase 'tightly-packed' is formed from the adverb 'tightly' (from 'tight') and the past participle 'packed' (from 'pack'), meaning 'packed in a tight manner.'

'Tightly' comes from 'tight' + '-ly', and 'packed' is the past participle of 'pack.' The phrase 'tightly-packed' emerged in modern English to describe things packed closely together.

Initially, it simply described the physical state of being packed tightly, and this meaning has remained consistent.