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1540s, a hybrid formed from practitian "practitioner" (c.1500, from French practicien, from Late Latin practicus "fit for action," see practice (v.)) on model of parishioner. Johnson has as a secondary sense "One who uses any sly or dangerous arts."
practitioner prac·ti·tion·er (prāk-tĭsh'ə-nər)n. One who practices medicine or an allied health profession.