before you can say Jack Robinson

At once, instantly. No one seems to be able to trace this term precisely or to discover the identity of Jack Robinson. Its earliest documented use was in 1778 in Fanny Burney’s Evelina (“I’d do it as soon as say Jack Robinson”). It appears in Dickens’s A Christmas Carol and Twain’s Huckleberry Finn. According to Francis Grose’s Classical Dictionary (1785), the original Jack Robinson was a gentleman who called on his neighbors so peremptorily that there was hardly time to announce him before he was gone.