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.
See also: before, can, jack, Robinson, say
Farlex Dictionary of Idioms.

before you could say Jack Robinson

OLD-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'.
See also: before, could, jack, Robinson, say
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • before (one) can say Jack Robinson
  • before you can say Jack Robinson
  • Robinson
  • jill off
  • jack all
  • jill it
  • Jack Tar
  • trade
  • decide between
  • decide between (someone or something)
References in classic literature
"Why," said he, "a magician could call up a lot of genies, and they would hash you up like nothing before you could say Jack Robinson. They are as tall as a tree and as big around as a church."
And before you could say Jack Robinson he had whipped out a whacking great revolver!"
Before you could say Jack Robinson, we had two Christian Churches in Britain -- and it was a bit of a mess-up .
BEFORE you could say Jack Robinson, a young golfer by the same name had won the regional rounds of the Golden Putter Competition and is heading for the national finals at Wentworth in July as North West Champion.