Slang is generally defined as informal language (words, phrases, and idioms) that is usually spoken or signed rather than written, peculiar to a particular group of people. It's often restricted to a particular context or group of people. These terms are often playful and trendy used by a group or generation.
A few examples of slang words in American Sign Language (ASL) are used by culturally Deaf signers below.
You may occasionally notice a slang "258" in Facebook comments within the Deaf community. Or, sometimes it's discreetly signed "258", which means, ASLized as "VERY INTERESTING". It's a language play in ASL among culturally Deaf signers who understand how to use it in specific right contexts. But, why "258". Well, watch the handshapes of the ASL phrase "VERY INTERESTING" again. And, you'd see the handshapes "2" for VERY and "5" -> "8" for INTERESTING.
Deaf children at a deaf school sometimes use "shaking L" or some kind of another code for the same concept in a classroom. While the children are doing whatever when their teacher is away from the classroom, one child would stay near the doorway to keep eye on the hallway. As soon as the teacher appears in the hallway, the child would shake "L" and everyone runs back to their seats.
Some slang words are only used by Deaf school kids (e.g. "shaking L"), some by a particular generation of Deaf people, some by Deaf families, some by Deaf people only that hearing signers shouldn't use (or it'd make them look awkward).