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

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "malfeasance":
   abuse, abuse of office, atrocity, bad policy, befoulment, breach,
   conversion, corrupt administration, crime, crime against humanity,
   criminal tendency, criminality, criminosis, deadly sin, debasement,
   defalcation, defilement, delinquency, dereliction, desecration,
   diversion, embezzlement, enormity, error, evil, evil courses,
   evildoing, failure, fault, feloniousness, felony, fouling,
   genocide, guilty act, heavy sin, impolicy, impropriety,
   indiscretion, inexpedience, inexpediency, inexpiable sin, iniquity,
   injury, injustice, lapse, lawbreaking, maladministration,
   malefaction, malpractice, malum, malversation, minor wrong,
   misadministration, misapplication, misappropriation, misconduct,
   misdeed, misdemeanor, misdirection, misdoing, misemployment,
   misfeasance, misgovernment, misguidance, mishandling,
   mismanagement, misprision, misprision of treason, misrule,
   misusage, misuse, mortal sin, neglect, negligence, nonfeasance,
   offense, omission, outrage, peccadillo, peccancy, peculation,
   perversion, pilfering, pollution, poor stewardship,
   positive misprision, profanation, prostitution, sin,
   sin of commission, sin of omission, sinful act, slip,
   thou scarlet sin, tort, transgression, trespass, trip,
   unutterable sin, venial sin, vice, viciousness, violation, wrong,
   wrong conduct, wrongdoing



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MALFEASANCE, contracts, torts. The unjust performance of some act which the
party had no right, or which he had contracted not to do. It differs from
misfeasance, (q.v.) and nonfeasance. (q.v.) Vide 1 Chit. Pr. 9; 1 Chit. Pl.
134.