The Collaborative International Dictionary of English v.0.48:

Malfeasance \Mal*fea"sance\, n. [F. malfaisance, fr. malfaisant
   injurious, doing ill; mal ill, evil + faisant doing, p. pr.
   of faire to do. See Malice, Feasible, and cf.
   Maleficence.] (Law)
   The doing of an act which a person ought not to do; evil
   conduct; an illegal deed. [Written also malefeasance.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

malfeasance
    n 1: wrongful conduct by a public official