More on Sworn at Highgate from Infoplease:

• Sworn at Highgate - Sworn at Highgate (See Highgate.) Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894 ...
• Highgate - Highgate has its name from a gate set up there about 400 years ago, to receive tolls for the bishop ...
• Dictionary of Phrase and Fable: S - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "S"