Green’s Dictionary of Slang

(US) an interrogation; also attrib.

1824		in J. Hall Letters from the West (1835) I 222: The torture of sweating [...] that is, of suspension by the arms, pinioned behind the back, brought a confession [DA].
1890		Boston Journal 10 Apr. 2/3: It is curious that in London and New York at the same time there should be a serious revolt against the ‘sweating system’ [DA].
1904		Cincinnati Enquirer 21 Oct. 4: He confessed, under sweating, that he broke into several offices [DA].
1953		J. Thompson Criminal (1993) 91: They usually are that way after a hard sweating.
1992		Bentley & Corbett Prison Sl. 94: Sweating may be done using force, threats or incessant demanding.