What was Big Willy Shakes going for?
Hold onto your hats, Shmoopers. It might come as a surprise that some scholars think "conclusion" meant "consummation" back in the day (source). So when Othello says "forgone conclusion," he's actually saying that Desdemona and Cassio consummated their relationship before Cassio was able to have a dream about it. Makes sense, right?
But Othello could also just be using the word in the way we understand it—to mean an outcome or a resolution. If that's the case, then he's saying that he's already made up his mind on the matter. It's been resolved before he even needs to see the evidence. He knows Dessie did it. That's his story and he's sticking to it.
The fact is nothing is a foregone conclusion in this play. We're not sure whether Desdemona and Othello have consummated their marriage. We're can't say for certain what the deal is with Othello being all too quick to assume his wife is cheating when she most certainly is not. And don't even get us started on Iago. We don't even know the reasons he's so set on ruining Othello in the first place.
Which raises a pretty important question: what's the deal with Othello thinking it's a foregone conclusion when it's not?
Othello and Desdemona are pretty happy and in love at the beginning of the play. So what the heck happens? We know that Iago manipulates Othello with his lies about Desdemona, but Iago never actually offers up any real proof of Desdemona's "affair," which suggests that Othello is pretty gullible—or that he doesn't trust his wife. He's going around town thinking things are concluded when they aren't. But why?
There are a couple of ways we can read Othello's eagerness to believe the worst about his wife. Some literary critics suggest that Othello believes that all women are inherently promiscuous. This seems to be the case when he says things like all men are "destined" to be cuckolded by their wives (3.3.42). Other critics argue that Othello begins to absorb the racist attitudes that surround him in Venice. In other words, Othello begins to believe that (1) he's not good enough for Desdemona because he's black and (2) as a black man, his relationship with his wife may "soil" her. So no wonder she's cheating, he thinks.
It's up to you how you interpret Othello's eagerness to condemn his wife, but one thing's for sure: it's not a foregone conclusion.