British slang

 Slang post update:

You can also say:

~"Tell me a slang expression"

~"Is this a slang word?" ("Is this word slang?")

I know that some words don't follow the same rules as other words, so the general idea with this one is that you use slang as a singular noun always, and then you add defining words to it if you need to clarify the sentence.

Logically, you SHOULD be able to say, "a slang" but if you do you will not sound native. I've never heard anyone native say this. 

However, you can say, "this is a type of slang" or similar. 

I hope that clarifies things.