The reason why 'Bob's your uncle' referring to nepotism and specifically to the career of this British prime minister, Balfour, did not appear in print until the 1930s is simple. [see the link above by Bedstor ]. Balfour, alive, could sue any publisher who alluded to it.British laws on defamation are weighted in favour of the subject, not the publisher, and the damages would have been crippling. To win he would only need to establish an innuendo that he had received favour early in his career. It would avail the defendants little that everybody thought so . The expression was alive and well,its origins and allusion well known, and living in the Foreign Office, inter alia, but nobody dare print it
Balfour died in 1930
The expression appears in print first in 1937 according to the link ( this may be a mistake for 1931, but either way,papers were free then to publish it).
Nobody can libel the dead. Their family has no remedy in defamation.
I was eating lunch with a fellow Aussie and told him how disgusting it is the way he piles his food on an upside down fork. He said "oh yeah? You Americans use your fork for a knife." At that point, I shut up.
. The expression was alive and well,its origins and allusion well known, and living in the Foreign Office, inter alia, but nobody dare print it 
