What do we mean when we say it today?
We're betting when you've heard this phrase used before, someone is actually talking about a spotless reputation. Not pulling the old bait and switch by saying you have a spotless reputation while really meaning everyone knows you're a murderer.
We thought so.
This phrase is usually used to mean just what it says, irony not included. Mowbray is outraged at being accused of something that Bolingbroke has no proof of. Just think of it as you did before you found out that Mowbray is actually a lying traitor.