fast scarlet R base noun Usage: often capitalized F&S : an orange-red crystalline amine CH3OC6H3(NO2)NH2 that is often sold in the form of its stabilized diazonium complex salt with zinc chloride and is used in producing azoic dyes; 4-nitro-ortho-anisidine — see dye table I (under Azoic Diazo 13)