Jargon, used playfully to prevent outsiders from intercepting the actual meaning. It is a contraction for "slandering", whereby colloqial words with meanings unknown to the one being slandered (but known to other people). This prevented legal action or apologies. It also allowed someone to insult an authority figure to his face without suffering punishment for it.
I don't use a great deal of modern day slang in my speech, but we all use slang created by shakespeare, eh?