The title 'Prime Minister' used to be a term of abuse. To be honest, depending on the holder of the office, plenty of people would maintain it still is. 

The first person of whom it was used as a real title in Britain, even if still not officially, was Robert Walpole. We've already met him, as a man who did rather well out of the South Sea Bubble, if more by good luck than by good judgement. That and the support he offered some powerful people in the wake of the scandal around the bubble, above all the king himself, helped him into office: he was seen as a safe pair of hands. 

A reputation he did much to nurture and generally deserved.

Illustration: Robert Walpole, First Earl of Oxford, by Arthur Pond, 1742

Music: Bach Partita #2c by J Bu licensed under an Attribution-NonCommercial-No Derivatives (aka Music Sharing) 3.0 International License