O monstrous, monstrous!
Nay, this was but his dream.
But this denoted a foregone conclusion.
Iago:Othello Act 3, scene 3, 427–431
'Tis a shrewd doubt, though it be but a dream,
And this may help to thicken other proofs,
That do demonstrate thinly.
Othello—the brave, romantic, and somewhat credulous Moor of Venice—has fallen under the evil spell of his ensign Iago. Here, Iago pretends that a certain Lieutenant Cassio has been dreaming—audibly—of Othello's chaste wife Desdemona. Othello believes that Cassio's alleged dreams must reenact the "foregone conclusion" of adultery. In Othello's mind, this speculation quickly "thickens" into proof that Desdemona has betrayed him.
When he coins this phrase, Othello seems to mean an adulterous act ("conclusion") which has preceded the dream, or what one editor calls a "previous consummation." But the matter isn't settled, no less in the original than in modern-day speech; "foregone conclusion" becomes only more confusing the more you think about it.
Today we use it to mean "predetermined outcome"—something anybody could have anticipated; but how we got from Othello to here is a mystery. Just as we now judge a person's dreams more as a wish for the future than a replay of the past, "conclusion" has come to refer to an inference of what will happen rather than an act that has already occurred.