I’m fairly certian it came from one of George Orwell’s novels he wrote in the mid 1900’s. I remember it being mentioned in either “Animal Farm” or “1984” (aka Nineteen Eighty-Four). Perhaps you could do a word search on either of those 2 novels.
It is a slogan (propaganda) by the government that the people should just be happy and now worry about who is running things and how they are running them.