"an inhabitant or member of the community of a parish," mid-15c., with -er (1), from earlier parishen "parishioner" (c. 1200), from Old French paroissien, parochien, from paroisse (see parish). The doublet form parochian was obsolete by 1700.
English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with," from Proto-Germanic *-ari (cognates: German -er, Swedish -are, Danish -ere), from Proto-Germanic *-arjoz. Some believe this root is identical with, and perhaps a borrowing of, Latin -arius (see -ary).
Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.
The use of -or and -ee in legal language (such as lessor/lessee) to distinguish actors and recipients of action has given the -or ending a tinge of professionalism, and this makes it useful in doubling words that have a professional and a non-professional sense (such as advisor/adviser, conductor/conducter, incubator/incubater, elevator/elevater).
c. 1300, "district with its own church; members of such a church," from Anglo-French paroche, parosse (late 11c.), Old French paroisse, from Late Latin parochia, paroecia "a diocese," an alteration of Late Greek paroikia "a diocese or parish," from paroikos "a sojourner" (in Christian writers), in classical Greek, "neighbor," from para- "near" (see para- (1)) + oikos "house" (from PIE root *weik- (1) "clan").
The sense development of the word in the early Church is unclear, perhaps it is from "sojourner" used as epithet of early Christians as spiritual sojourners in the material world. In early Church writing the word was used in a more general sense than Greek dioikesis, though by 13c. they were synonymous. It replaced Old English preostscyr, literally "priest-shire." In Great Britain (from the 1630s), some southern American colonies, and Louisiana it has been the name for a purely civil division for purposes of local government, with boundaries originally corresponding to an ecclesiastical parish.
1540s, "one who acquires knowledge from actual practice;" a hybrid formed from practitian "practitioner" (c. 1500; see practician), with redundant ending on model of parishioner. Meaning "one engaged in the actual practice of an art or profession" is from 1550s. Johnson has as a secondary sense "One who uses any sly or dangerous arts" (compare practice (n.)). A general practitioner originally was "someone who practices both medicine and surgery."