tosylPronunciation: /ˈtəʊsʌɪl, -sɪl/Definition of tosyl noun [as modifier] Chemistry of or denoting the toluene-4-sulphonyl radical SO2C6H4CH3, used in organic synthesis: tosyl chlorideOrigin: 1930s: from German, from to(luol) and s(ulphon)yltosyl in other Oxford dictionaries Definition of tosyl in the US English dictionary