stuff has 26 distinct synonyms spread across 7 of its 14 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is unstuff.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
informal terms for personal possessions
“did you take all your clobber?”
senseless talk
“don't give me that stuff”
unspecified qualities required to do or be something
“the stuff of heros”
information in some unspecified form
“it was stuff I had heard before”
a critically important or characteristic component
“suspense is the very stuff of narrative”
Synonyms, antonyms and the broader and narrower relations for stuff come from Princeton University’s WordNet. Full sources and licences.
overeat or eat immodestly; make a pig of oneself
“She stuffed herself at the dinner”
treat with grease, fill, and prepare for mounting
“stuff a bearskin”