A collective adjective is a type of nominal adjective, which means it doesn’t actually function as an adjective at all. Like other nominal adjectives, collective adjectives are words normally used as adjectives that are functioning as nouns. And like collective nouns, collective adjectives are used to refer to groups of nouns that are described by the characteristic indicated by the nominalized adjective.
The elderly deserve our respect and consideration.
Normally elderly is used as an adjective to describe a noun, but here it stands alone as a noun to refer to elderly people as a group. The sentence could be reworded to say, “Elderly people deserve our respect and consideration.” The same idea applies to the following examples:
We must defend the weak and helpless.
The homeless will be provided for at the shelter.
Fortune favors the bold.
Adjectives used for nationalities may often double as collective adjectives to refer to the people of a country. However, not all adjectives of nationality may be used this way. For example, when speaking of American or Korean people, we would have to refer them as Americans and Koreans. Also take note that collective adjectives of nationality are not only collective nouns; they are also proper nouns and must be capitalized.
The Japanese are famous for their politeness and innovative spirit.
The British once ruled a vast empire.
The Spanish enjoy a refreshing siesta in the afternoon.