What was Big Willy Shakes going for?
This passage isn't very straightforward or clear cut, so let's break it down. Shrift is just a fancy word for a confession. And when something is short, it's quick. So, you might have guessed, that short shrift is a quick confession.
Ratcliffe is telling Hastings to make a fast confession because he'll be dead soon. Gulp. He's not kidding. Hastings is bumped off shortly after this. We're betting Ratcliffe says this with a touch of irony. He doesn't really care if Hastings makes a confession or not. But it's a quick and dirty way of making a point, as in, "Hint, hint. You'll be dead soon, so we better get on with it."
When Shakespeare came up with this phrase, people regularly confessed before execution. This was a way for the executioner to not feel guilty about killing someone. Even though Ratcliffe doesn't feel bad about killing Hastings, he says this as a way of telling the audience what's going to happen. We're clued in and ready to shield our eyes.