Guff is foolishness — it's when you behave or speak in an meaningless or objectionable way. You might dismiss a controversial article by calling it a bunch of guff.
Sometimes guff is defined as "lies," but it can also be simply ridiculous ideas or speech. Your teacher might dismiss your excuse for being late to class by saying, "That story's just a bunch of guff!" The informal guff gets its sense of "empty talk" from its original meaning, "puff of air." An unrelated but interesting kind of guff is the Scottish meaning, "an unpleasant smell."