Since slang (and thieves' cant) originated in the United Kingdom, it has more slang words than most other languages, partially because it has had more time to develop new words, and partially because it is the unofficial main language of the internet.

The Wikipedia page alone already has 300 wonderful words and phrases, and these are only a small margin of the amount of words out there. 

Some words are used throughout Britain, like knackered for exhausted for example, and some are used in smaller regions. Of course, the nations of the UK all have their own slang words, as does London, which has many varieties. 

Countries that used to be part of the British Empire might also use some of these, but use their own as well to reflect their own cultures. 

British slang has been the subject for many books, including a seven volume dictionary from 1890. 

Some well known British slang words include:

Nob - Person of high social status, snob.

Skive - Feigning illness to get out of going to work or school. ( Which returns us to the word of the day for the Third of April)

Tosh - Nonsense

Bloody - Damn

Blimey! - My goodness!

Bee's knees - Awesome

So how is it over in the United States?