Before you can say Jack Robinson
Definition 1 of 1
Pro Tip 1/3Speed Implication
It suggests something happens so quickly it's almost surprising. 
She accepted the job offer before you can say Jack Robinson.
Pro Tip 2/3Colloquial Usage
It's an informal saying, used more in casual conversations than in formal writing. 
He packed his bags and was gone before you can say Jack Robinson.
Pro Tip 3/3Hyperbole Alert
It's an exaggeration, as no action literally takes that short amount of time. 
The cookies sold out before you can say Jack Robinson.