Symbolism, Imagery, Allegory
As Brachiano loses his mind from poisoning, he has this little exchange with Flamineo:
Brach. Why, there,
In a blue bonnet, and a pair of breeches
With a great cod-piece: ha, ha, ha!
Look you, his cod-piece is stuck full of pins,
With pearls o' th' head of them. Do you not know him?
Flam. No, my lord.
Brach. Why, 'tis the devil.
I know him by a great rose he wears on 's shoe,
To hide his cloven foot. I 'll dispute with him;
He 's a rare linguist. (5.3).
This might just be the ravings of a madman. But it might include a deeper layer of meaning, as well. It seems significant that the devil is wearing a cod-piece stuck with pins—the Duke's murders were sexually motivated, and to have pain associated with sexuality makes a lot of sense. He's someone who used his sexual desires to ruin the lives of other people, and now, confronting an image of damnation, he sees Satan wearing this unusual piece of leisure clothing.
(Note: Men in the 15th and 16th Century actually did regularly wear cod-pieces… as a, uh, fashion statement… Okay—the point was to make certain parts of their bodies look bigger. Real talk.)