fun, to quote the free dictionary:
The use of fun as an attributive adjective, as in a fun time probably originated in a playful reanalysis of the use of the word in sentences such as It is fun to ski, where fun has the syntactic function of adjectives such as amusing or enjoyable. The usage became popular in the 1950s and 1960s, though there is some evidence to suggest that it has 19th-century antecedents, but it can still raise eyebrows among traditionalists. The day may come when this usage is entirely unremarkable, but writers may want to avoid it in more formal contexts.
It is evident that fun can be expressed in a funny way. The tag fun is hopefully used to arouse amusement among the audience of the published jocularity. The relative uselessnes of the work is probably a good measure of its success.