Green’s Dictionary of Slang

(US) a trick, a hoax.

1835		W. Otter Hist. of My Own Times (1995) 74: By way of a slope, I also formed an intimacy with one of his daughters.
1880		Bulletin (Sydney) 22 May 4/4: A certain fashionable Melbourne editor who ‘does Collins-street’ with a wealthy young heiress, has of late been much embarrassed by finding that a good many people are aware that he left a nice-looking wife on the ‘Pacific slope.’ The question is, was the ‘slope’ altogether ‘Pacific’.