'monthly' originates from Old English, specifically the word 'mōnathlic,' where 'mōnath' meant 'month' and '-lic' meant 'like or pertaining to.'

'mōnathlic' transformed into the Middle English word 'monethly,' and eventually became the modern English word 'monthly.'

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