Green’s Dictionary of Slang

swear n.

an oath.

c.1635		W. Cartwright Ordinary IV iv: Gull’d by my swear, by my swear gull’d.
1691		‘Lay your Reason’ in Political Ballads in Wilkins (1860) II 31: You must either take the swear, or starve, or lose your station.
1702		T. Brown Letters from the Dead to the Living in Works (1760) IV 79: [He has] faced about to the Right, and taken the Swear.
1887		St James’s Gazette 4 June n.p.: It is a dreadful thing to say, but I felt that if I didn’t utter a big swear at that moment something would happen [F&H].
1906		Sun. Times (Perth) 21 Oct. 4/8: Excuse me while I say a swear.
1918		C.J. Dennis ‘Sore Throat’ in Backblock Ballads 59: To use the fancy swears I hear / Comes natural as sinkin’ beer.