before (one) can say Jack Robinson

Very quickly or suddenly. (The true identity of Jack Robinson is unknown.) I'm not far from your house, so I'll be there before you can say Jack Robinson. Before I could say Jack Robinson, Tom had leapt to his feet and rushed out of the room.

Fig. almost immediately. (Often found in children's stories.) And before you could say Jack Robinson, the bird flew away. I'll catch a plane and be there quicker than you can say Jack Robinson.

Also, quicker than you can say Jack Robinson. Almost immediately, very soon, as in I'll finish this book before you can say Jack Robinson. This expression originated in the 1700s, but the identity of Jack Robinson has been lost. Grose's Classical Dictionary (1785) said he was a man who paid such brief visits to acquaintances that there was scarcely time to announce his arrival before he had departed, but it gives no further documentation. A newer version is before you know it, meaning so soon that you don't have time to become aware of it (as in He'll be gone before you know it).

COMMON If you say that something happened before you could say Jack Robinson, you mean it happened very suddenly and quickly. The pair of them were out of the door and down the steps before you could say Jack Robinson. Note: People sometimes change Jack Robinson to a word or expression that relates to the subject they are talking about. She was on the phone to New York before you could say long-distance. Note: The identity of Jack Robinson is unknown and the name might have become popular because both elements were once extremely common. The phrase appears in the novel `Evelina' (1778) by Fanny Burney: `I'll do it as soon as 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.