What do we mean when we say it today?
Shakespeare might have been talking about doing something else (something a bit more scandalous, say) when he coined this phrase, but today, it's all about the inevitable. People say this when they know something is decided before evidence is even considered.
You might think this phrase is only used in courtrooms or really dramatic courthouse TV shows (we're looking at you, Law and Order). But it's also used in every day life. The president's run for reelection. The football team's win. The next Avengers movie. A shirtless scene with Ryan Gosling. All of these are foregone conclusions—things people are certain will happen. No one needs to tell us. We just know they will eventually take place.