Monthly means happening once every month. A monthly meeting, paid monthly, a monthly magazine.
It works as an adjective and as an adverb: the rent is due monthly. As a noun, a monthly is a magazine published once a month. Note bimonthly, which is genuinely ambiguous and means both twice a month and every two months, so careful writers avoid it entirely; and that monthly payments, monthly instalments and monthly salary are among the fixed phrases of ordinary financial life.
1adjectivemonthlyHappening once every month.
“Our monthly phone bill has gone up again.”
2nounmagazineA magazine published once a month.
“She writes a gardening column for a popular monthly.”
monthly is a frequency.
Old English monathlic is month plus -ly, and month comes from the moon. Daily, weekly, monthly and yearly are built identically and all four double as adverbs.
Proto-Germanic
*mēnōþlīkaz — monthly
Proto-West Germanic
*mānōþlīk
Old English
mōnaþlīċ — monthly
Middle English
*monethly
English, today
monthly
From month + -ly, perhaps via Middle English monethly (attested only adverbially), from Old English mōnaþlīċ (“monthly”), from Proto-West Germanic mānōþlīk, from Proto-Germanic mēnōþlīkaz (“monthly”). Cognate with stq moundelk (“monthly”), German monatlich (“monthly”), Danish månedlig (“monthly”), Swedish månatlig (“monthly”), Norwegian Bokmål månedlig (“monthly”), Icelandic mánaðarlegur (“monthly”).
Etymology from Wiktionary, CC BY-SA