-ly Also -ily.
Forming adjectives and adverbs.
[Old English adjectival ending -lic or adverbial ending -līce, of Germanic origin.]
One set of adjectives indicate a quality of some kind. They usually derive from nouns, though some are based on other adjectives: beastly, cleanly, cowardly, deadly, evenly, gladly, heavenly, neighbourly, rascally, scholarly, weakly. Many can also be adverbs, though some, such as miserly and unruly, are restricted to adjectival use. A second set indicate something recurring at an interval: hourly, daily, weekly, monthly, quarterly, yearly; these can all act as adverbs.
The -ly ending most characteristically marks adverbs, and is the usual way of forming them from adjectives. A very large number exist, of which a few examples are apparently, commonly, electrically, essentially, foolishly, freely, greatly, immediately, occasionally, surprisingly, thankfully, and wrongly.
Adverbs formed from adjectives that end in y preceded by a consonant replace the y by a i to make the ending -ily: busily, drowsily, floppily, hastily, lazily, primarily, wittily.