Never heard the expression. But 'hog' was a term used by bikers and Hell's Angles for their bikes; 'on a hog' and 'on a hog's back' were terms for riding. There was even an album by the risible Black Oak Arkansas called High on a Hog. Also the Hog's Back is a ridge of the North Downs that the A3 runs along near Guildford. Probably not the answer you wanted....?