Back in the day when people still made sculptures, some artists made mistakes. To correct these mistakes when making a sculpture of stone, they would use a colored wax to fill in the mistake. A truly great sculpture was described as being "without wax." This term went on to be used for anything that was authentic or flawless. It is also the root of the word sincerely. Sin - without and Cera - wax. Also a smooth compliment to give to a beautiful and intelligent woman (dumb women would get confused by the sculpture reference) or a smooth way to end a letter to prove your sincerity.