Most English compound nouns are endocentric. This means that the central meaning of the compound is carried by the head. The head of English compounds is on the right. In some cases, though, the central meaning isn't carried by the head. Such compounds are called exocentric.
The English compounds newspaper, textbook, classroom, handbag and bookstore are examples of endocentric compounds. The central meaning of each one is carried by the second word of the compound. For example, a newspaper is a kind of paper and a textbook is a kind of book. These compounds are hyponyms of their heads.
With exocentric compounds, however, the central meaning of the compound isn't conveyed by the head. The meaning is external to the literal meaning of the compound. Examples of exocentric compounds include scarecrow, redhead, pickpocket, showoff and paperback. They're called exocentric because a scarecrow isn't a kind of crow and a redhead isn't a kind of head. A scarecrow is an object designed to scare not only crows but all birds, a redhead is a person with red hair, a pickpocket is a person who steals from people's pockets, a showoff is a person who shows off, and a paperback is a book which is paper-bound. Notice that the word showoff doesn't contain a noun but rather a verb and a particle. When the words show and off are written separately, it's actually a verb.
Though compound nouns are usually endocentric, a large number are exocentric. With exocentric compounds, the central meaning isn't carried by the head but is external to the compound. Compound nouns usually consist of at least one noun, but not always. The compound showoff is an example of a compound noun which has no noun. Many phrasal verbs can also function as compound nouns such as takeout, pickup, breakup, breakdown and takeoff.