• noun (in phrase Bob’s your uncle) Brit. Informal an expression signifying the simplicity of completing a task
This is a catchphrase which seemed to arise out of nowhere and yet has had a long period of fashion and is still going strong. It’s known mainly in Britain and Commonwealth countries, and is really a kind of interjection. It’s used to show how simple it is to do something: "You put the plug in here, press that switch, and Bob’s your uncle!".
The most attractive theory—albeit suspiciously neat—is that it derives from a prolonged act of political nepotism. The Victorian prime minister, Lord Salisbury (family name Robert Cecil) appointed his less than popular nephew, Arthur Balfour, to a succession of posts. As the story goes, the consensus among the irreverent in Britain was that to have Bob as your uncle was a guarantee of success, hence the expression. Since the very word nepotism derives from the Italian word for nephew (from the practice of Italian popes giving preferment to nephews, a euphemism for their illegitimate sons), the association here seems more than apt.
There is a 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.
Assitant!Jo informs me that all the cool kids are actually saying:
Bob's your uncle, Fanny's your aunt!
If you want to use the phrase in the Cockney vernacular, the your should be pronounced yer