before you could say Jack Robinson
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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
before you could say Jack RobinsonOLD-FASHIONED
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'.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012