What does Lady Macbeth say that Macbeth's actions are custom?

She says:

Think of this, good peers,

But as a thing of custom: 'tis no other;

Only it spoils the pleasure of the time.

She is trying to laugh off his insane behaviour. Totally unsuccessfully, I might add. She says, "What, scream and yell at things that are not there? Oh, don't worry about that, he does that all the time." Yeah, right.