Malfeasance Definition: Doing something which is illegal. Doing something which is illegal. In Proksch v. Bettendorf, 218 Iowa 1376 (1934), the Supreme Court of Iowa adopted these words: "Nonfeasance ... means the omission of an act which a person ought to do. "Misfeasance is the improper doing of an act which a person might lawfully do. "Malfeasance is the doing of an act which a person ought not to do at all." Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!