'nightly' originates from Old English, specifically the word 'nihtlic,' where 'niht' meant 'night' and '-lic' was a suffix meaning 'like' or 'pertaining to.'

'nihtlic' transformed into the Middle English word 'nightlich,' and eventually became the modern English word 'nightly.'

Initially, it meant 'pertaining to the night,' and this meaning has largely remained the same in modern usage.