There are no rules that can account for the meaning of a semantically opaque compound. These compounds are said to be lexicalized. They cannot be processed by the mechanisms of meaning extraction because of extralinguistic associations which depend on the speaker's background, knowledge, or education. Whereas the meaning of acid rain , rain which contains harmful acid, can be easily inferred, it is impossible to deduce the meaning of acid house , which is a style of popular music, by just knowing the meaning of acid and house. The element acid probably refers to the drug LSD, and house is the abbreviation of Warehouse, which is the name of a nightclub in Chicago where this form of dance music originated.
Fern bar is another good example.
A fern bar referred to a bar in which expensive drinks were served to upwardly-mobile customers. However, later on, a fern bar still referred to an expensive bar, but there were not necessarily ferns in the decoration. Derivation or affixation , the addition of prefixes and suffixes to a base, has also been very common from the earliest stages of English. There are many traditional affixes that are still productive and are used to generate new words. Although it is sometimes difficult to process the meaning of words that have been formed by affixation, new ones are generally transparent.