before 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).
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.