pron·to·sil \ˈpräntəˌsil\noun (-s) Etymology: German : any of three sulfonamide drugs: a.orprontosil rubrum: a red azo dye H2NSO2C6H4N . NC6H3(NH2)2 that was the first sulfa drug tested clinically b.orprontosil album: sulfanilamide c.orprontosil soluble: azosulfamide