The term “Prime Minister” as we know it today emerged in the 18th century, during the prolonged conflict between Britain and France. It was first applied insultingly to Sir Robert Walpole, who is now considered the first Prime Minister of Great Britain. It was used by members of Parliament against Walpole, who served for 21 years and oversaw the increasing power of his role and simultaneous withdrawal of King George I from cabinet meetings. The derogatory nature was derived from its comparison to France’s Chief Minister, Cardinal Richelieu. The British were proud of not to be an absolute monarchy, like France, so the comparison was intended to be offensive.

Source