"church law," Old English canon, from Old French canon or directly from Late Latin canon "Church law," in classical Latin, "measuring line, rule," from Greek kanon "any straight rod or bar; rule; standard of excellence," perhaps from kanna "reed" (see cane, n.). Taken in ecclesiastical sense for "decree of the Church." General sense of "standard of judging" is from c.1600. Harold Bloom writes that "The secular canon, with the word meaning a catalog of approved authors, does not actually begin until the middle of the eighteenth century ...." ["The Western Canon," 1994]. Related: Canonicity.

"clergyman," c.1200, from Anglo-French canun, from Old North French canonie (Modern French chanoine), from Church Latin canonicus "clergyman living under a rule," noun use of Latin adjective canonicus "according to rule" (in ecclesiastical use, "pertaining to the canon"), from Greek kanonikos, from kanon "rule" (see canon, n.1).