These days, when people talk about a “foregone conclusion” they mean something is a given: it is inevitable, it will happen, it may safely be assumed. As certain as it sounds, it is still a statement of conjecture about an event that is yet to occur.

When Shakespeare had Othello speak those words in Act 3, Scene 3 of the play that bears his name, it had quite the opposite meaning.
In this scene, Iago is manipulating Othello’s thoughts and making him believe that Desdemona has cheated on him. 

Othello says, “But this denotes a foregone conclusion:  Tis a shrewd doubt, though it be a dream.” 

Here, he is speaking of the adultery between Desdemona and Cassio as something that he is certain has already happened. This gives the phrase “foregone conclusion” the opposite meaning to that which it holds today. 

This, and statements such as “I’ll tear her all to pieces” and “O blood, blood, blood!” are evidence that Othello has already made up his mind about the guilt of his wife and former second-in-command. 

The scene ends with Othello swearing his loyalty to Iago and thinking of ways to kill Desdemona. Charming, I know.