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.
1977		Fidrych & Clark No Big Deal 20: [note] ‘My agent went through it [i.e. a manusript] and said there were too many swears’.
2007		B. Coleman Check the Technique 103: ‘That’s really the only track with any swears on it. The more hostile you are with the situation, I guess the fouler your language is gonna be’.

In phrases

put the swear on (v.)

to curse.

1690

‘Teague’ Teagueland Jests I 65: He will put de Shwear upon de Scots-man for shpeaking Treason [...] Teague returned to rave and swear, Bee de mash [...] he will put de Shwear upon all of dem.