It goes "Unaccustomed as I am to public speaking I would like to say...", I think it was a good thing to start a speech with and has now become a cliche.
I seem to recall something to do with the Prince Regent during the reign of King George III.
This may be more to do with a Blackadder sketch than history though!
It was featured in an episode of Blackadder the Third (1987) called "Sense and Senility". The Prince Regent, played by Hugh Laurie, was being taught how to give a speech by two stage actors.