These sorts of adjectives are created by English speakers and writers to supply a concept for which no single word suffices. They mean that the the noun modified possesses the qualities of the second word to the degree of the first word. For instance, 'long-lived' means that something has existence for a lengthy period of time; 'It was a long-lived debate' means that it was an arguement that lasted for a while. Some of these compounds are used so frequently that you see them without the hyphen. For example, short-sighted means that someone posses the ability to see but not very far; it is often written 'shortsighted.'
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