haiku directive is supplied by the haiku plugin.
This directive allows inserting a randomly generated haiku into a wiki page. Just type:
[[!haiku hint="argument test"]]
The hint parameter can be omitted, it only provides the generator a hint of what to write the haiku about. If no hint is given, it might base it on the page name. Since the vocabulary it knows is very small, many hints won't affect the result at all.