matter, material, articles, or activities of a specified or indeterminate kind that are being referred to, indicated, or implied.
a pickup truck picked the stuff up
the basic constituents or characteristics of something or someone.
Healey was made of sterner stuff
woolen fabric, especially as distinct from silk, cotton, and linen.
her dark stuff gown
(in sports) spin given to a ball to make it vary its course.
I think Greinke's stuff will get better, it got better as last year went along.