Ask anyone to name a distinguishing characteristic of an adverb, and the reply might be that such a word ends with -ly. Although that is often true, some adverbs, such as fast, lack the ending. For this reason, they are known as flat adverbs. In addition, many words ending in -ly aren’t adverbs.
Many adjectives end in -ly (which means—and is cognate with—“like”), including some that are also adjectives in their “flat” form. For example, dead and deadly are both adjectives. Deadly may look like an adverb, but one cannot say that one person stared deadly at another person; a correct treatment would be to employ deadly as an adjective and use the noun form of stared: “He gave her a deadly stare.” A more prominent error is to use timely as if it were an adverb, as in “She was instructed to complete the report timely.” But it is an adjective, and should be treated as such, as in “She was instructed to complete the report in a timely manner.”
Some words ending in -ly serve as both adjectives and adverbs, such as friendly, likely, and stately. (Other adjectives that look like adverbs but serve only the former function include costly and worldly.) Others, which do not have root words, include early and ugly (both adjectives and adverbs) and burly and grisly (which are only adjectives). Occasionally, an adjective ending in -ly can be converted into an adverb by changing the ending to -lily, but words like friendlily and uglily are rare in writing and almost unheard of in speech.
Many adjectives are merely nouns referring to people and with -ly attached, as in the case of brotherly, neighborly, and scholarly, or pertaining to time (for example, monthly) or direction (for example, northerly). Note that many other nouns also end in -ly, such as assembly (based on the verb assemble) and bully (where the ending is a result of the pronunciation of the source word from another language), and some verbs do, too, such as comply and reply.