"Bah, humbug" is a famous quotation from Charles Dickens' book "A Christmas Carol"; it is said by the main character, Scrooge, who is a mean, embittered man who does not believe in the spririt of Christmas. "Humbug" is another word for nonsense; "bah" is simply an exclamation, added for emphasis.You would have to read the book to really understand the expression. ("Boo, humbug" doesn't mean much at all.)