Brewer's: Jack Robinson
Before you can say Jack Robinson. Immediately. Grose says that the saying had its birth from a very volatile gentleman of that name, who used to pay flying visits to his neighbours, and was no sooner announced than he was off again; but the following couplet does not confirm this derivation:
A warke it ys as easie to be done As tys to saye Jacke! robys on.
An old Play, cited by Halliwell: Arch. Dict.