Green’s Dictionary of Slang

1. an oath, a ‘swear’.

1910		‘Doss Chiderdoss’ ‘Great Expectations’ Sporting Times 1 Oct. 1/3: Those relatives, we well can understand, / Breathed a ‘Lord Mayor’ when they found that the anticipated ‘dust’ / Took the form of non-negotiable sand.
1969		S.T. Kendall Up the Frog.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.

2. a chair.

1946		St. Vincent Troubridge ‘Some Notes on Rhyming Argot’ in AS XXI:1 Feb. 46: here and there. A chair. (Origin uncertain, but probably English.) This is doubted. Lord Mayor was always used.
1960, 1975		Wentworth & Flexner DAS.