I only see evidence of three occasions in which the word "stuff" is used in the text. In each of these quotes, the word is used as a noun, meaning "matter, material, articles, or activities of a specified or indeterminate kind that are being referred to, indicated, or implied".
My lord, there was no such stuff in my thoughts. (ACT II, Scene II)
Ay, lady, it was my word.
[Lifts up the arras and sees Polonius.]
Thou wretched, rash, intruding fool, farewell!
I took thee for thy better. Take thy fortune.
Thou find'st to be too busy is some danger.
Leave wringing of your hands. Peace! sit you down
And let me wring your heart; for so I shall
If it be made of penetrable stuff;
If damned custom have not braz'd it so
That it is proof and bulwark against sense. (Act III, Scene IV)
Break not your sleeps for that. You must not think
That we are made of stuff so flat and dull
That we can let our beard be shook with danger,
And think it pastime. You shortly shall hear more.
I lov'd your father, and we love ourself,
And that, I hope, will teach you to imagine-
[Enter a Messenger with letters.]
How now? What news? (Act IV, Scene VII)