1
general::
Phrase(s): before you can say Jack Robinson [and] quicker than you can say Jack Robinson
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.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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).
American Heritage Idioms