In Mandarin, for example, in order to write a slang word, you would combine two dictionary-existing characters (Chinese written characters, also known as "Han zi") to make a slang word.

The way I see it, although the slang word is an addition to the known vocabulary, it is still made up of known Chinese characters.

However, based on what your saying, using sign language, in order to learn the slang word equivalent to a known word, you have to learn a whole new sign?