True loveliness (is a lasting quality). The complete thought is a line from John Keats’s great poem, “Endymion” (1818), and continues, “Its loveliness increases; it will never pass into nothingness.” The thought is hardly original, but the expression caught on. In the twentieth century it began to be shortened and used simply to describe something lovely, often ironically. Eric Partridge mentioned a play on it made by “flappers,” fashionable young women of the post–World War I era: “A thing of beauty is a boy forever.” Today one is apt to say it of, for example, an elaborately decorated cake.