phe·neth·i·cil·lin \fə̇¦nethə¦silə̇n\noun (-s) Etymology:phen- + eth- + penicillin : a semisynthetic penicillin C17H20N2O5S administered orally in the form of its potassium salt and used especially in the treatment of less severe infections caused by bacteria that do not produce penicillinase