tough has 27 distinct synonyms spread across 10 of its 12 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 tender.
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.
not given to gentleness or sentimentality
“a tough character”
very difficult; severely testing stamina or resolution
“a rugged competitive examination”
feeling physical discomfort or pain (`tough' is occasionally used colloquially for `bad')
“my throat feels bad”
resistant to cutting or chewing
making great mental demands; hard to comprehend or solve or believe
“a baffling problem”
someone who learned to fight in the streets rather than being formally trained in the sport of boxing
Synonyms, antonyms and the broader and narrower relations for tough come from Princeton University’s WordNet. Full sources and licences.