"There is another big problem: the phrase isn't recorded until 1937, in Eric Partridge's Dictionary of Slang and Unconventional English. Mr Partridge suggested it had been in use since the 1890s, but nobody has found an example in print. This is surprising. If public indignation or cynicism against Lord Salisbury's actions had been great enough to provoke creation of the saying, why didn't it appear — to take a case — in a satirical magazine of the time such as Punch?

A rather more probable, but less exciting, theory has it that it derives from the slang phrase all is bob, meaning that everything is safe, pleasant or satisfactory. This dates back to the seventeenth century or so (it's in Captain Francis Grose's Dictionary of the Vulgar Tongue of 1785). There have been several other slang expressions containing bob, some associated with thievery or gambling, and from the eighteenth century on it was also a common generic name for somebody you didn't know. Any or all of these might have contributed to its genesis."