Well, I declare!

That is a phrase which I associate with American matrons expressing surprise; the OED gives a quotation dated 1839 from Kavanagh by Longfellow : Well, I declare! If it is not Mr. Kavanagh! The OED also notes that “I declare” can be used as a mere asseveration ("emphatic confirmation of a statement; a word or phrase used to express confirmation; an oath").

Here is another quotation, not used by the OED; it is from Stanley Holloway's monologue My Word, You Do look Queer! in which a poor chap is told how ill he looks by everyone he meets:
By gosh, who'd have thought it? Well, well, I declare,
I'd never have known you except for your hair!

In February 2000 the exact meaning of the verb “to declare”, as defined not by the OED but by its great predecessor, Samuel Johnson’s Dictionary, became a matter of some importance in a federal lawsuit. Seventeen Congressmen brought an action arguing that President Bill Clinton had failed to obtain a “declaration of war” before launching 4,500 air strikes against Yugoslavia, and had thereby deprived them of their right under the US Constitution to choose whether to commit their country to war or to refuse their consent. One of the issues at stake was the exact meaning of the words declare and war, and the decision was taken to consult the dictionary which would have been the standard authority when the Constitution was drawn up in 1787, which was Johnson’s. His Dictionary has been cited many other times in debates over the Constitution in the American courts.

When Noah Webster sent a copy of the second edition of his own work An American Dictionary of the English Language to Queen Victoria he said to the diplomat who carried it to her “Our common language is one of the ties that bind the two nations together” (George Bernard Shaw later gave a different slant to that remark).

Webster admired Johnson but loathed his Dictionary; he would not have been happy had he known that it will have a role to play in American law as long as the American Constitution remains intact.

Acknowledgements to Henry Hitchings' Dr Johnson's Dictionary