Equiv. to urban dictionary?

Just wondering if there's some kind of slang dictionary, a la urban dictionary etc, out there for when you see a term a normal dictionary won't define?

(I don't mind if the definitions are in Japanese, if one exists)

Alc is probably the most comprehensive E/J dictionary you can find, at least on-line.
http://www.alc.co.jp/

When you can't find words in a dictionary, your second option would be to look up examples using an internet search engine.

Yahoo is easy to use, when it comes to language. For example, if you need to know the meaning or usage of the Japanese word 言葉, look up 言葉 on yahoo Japan and it will give you many usage examples. Then you can guess the meaing and get the feeling of it. If you need to look up an English word, look it up on yahoo USA or yahoo UK&Ireland. It would be better if you pick up two or three words from the context and put "" quotation marks around it.

Of course, there are always various slang dictionaries like this one;
http://zokugo-dict.com/

By looking words up on yahoo, you often end up on pages of these kind of dictionaries.

Much obliged.