THE TRUTH IS THE ONE THING THAT NOBODY WILL BELIEVE

The above quotation is another famous Shavian quip that needs sourcing. In this case, we find these words in a conversation between Napoleon and The Lady in The Man of Destiny (1898)

• 
  
• NAPOLEON. Aha! I thought so: a little romance to get the papers back. (He throws the packet on the table and confronts her with cynical goodhumor.) Per Bacco, little woman, I can't help admiring you. If I could lie like that, it would save me a great deal of trouble.
• LADY (wringing her hands). Oh, how I wish I really had told you some lie! You would have believed me then. The truth is the one thing that nobody will believe.

I suppose we can put this maxim in the same folder as "all great truths begin as blasphemies" and Shaw's alleged dramatic technique of saying things "with the utmost levity" although "the real joke was that he was in earnest." Truth be told, we wouldn't believe him anyway.