fat has 17 distinct synonyms spread across 6 of its 9 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 thin.
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.
having an (over)abundance of flesh
“he hadn't remembered how fat she was”
a soft greasy substance occurring in organic tissue and consisting of a mixture of lipids (mostly triglycerides)
“pizza has too much fat”
1 of the 9 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for fat.
Synonyms, antonyms and the broader and narrower relations for fat come from Princeton University’s WordNet. Full sources and licences.
a kind of body tissue containing stored fat that serves as a source of energy; it also cushions and insulates vital organs
“fatty tissue protected them from the severe cold”
excess bodily weight
“she disliked fatness in herself as well as in others”