Green’s Dictionary of Slang

to withdraw from an undertaking, e.g. an engagement to be married.

1749		Fielding Tom Jones (1959) 520: Propose marriage [...] and she will declare off in a moment.
1766		O. Goldsmith Vicar of Wakefield (1883) 82: No, I declare off; I’ll fight no more.
1791		Gouv. Morris in Sparks Life and Writ. (1832) III 19: I contrived to get clear by declaring off from being a candidate.
1876		‘George Eliot’ Daniel Deronda (1967) 830: When it came to the point, Mr. Haynes declared off, and there has been no one to take it since.