1540s, "a statement generally accepted," a literal translation of Latin locus communis, itself a translation of Greek koinos topos "general topic," in logic, "general theme applicable to many particular cases." See common (adj.) + place (n.). Meaning "memorandum of something that is likely to be again referred to, striking or notable passage" is from 1560s; hence commonplace-book (1570s) in which such were written down. Meaning "well-known, customary, or obvious remark; statement regularly made on certain occasions" is from 1550s. The adjectival sense of "having nothing original" dates from c. 1600.
The sense evolved from the word's use in designating things like ovens or mills that were used in common by serfs, or else in reference to compulsory feudal military service; in either case it was generalized in French through "open to everyone" to "commonplace, ordinary," to "trite, petty." The word was earlier used in English with a sense "pertaining to compulsory feudal service" (1753). Related: Banalize; banalization.
The earliest use of the word in English was early 15c., a separate borrowing in the academic sense "of the trivium" (the first three liberal arts -- grammar, rhetoric, and logic); from Medieval Latin use of trivialis in the sense "of the first three liberal arts," from trivium, neuter of the Latin adjective trivius "of three roads, of the crossroads." Related: Trivially. For sense evolution to "pertaining to useless information," see trivia.
1812, "dullness, insipidity of thought, triteness," from French platitude "flatness, vapidness" (late 17c.), from Old French plat "flat" (see plateau (n.)); formed on analogy of latitude, etc. Meaning "a flat, dull, trite, or commonplace remark," especially a truism uttered as if it were a novelty, is recorded from 1815. Related: Platitudinous (1862). Hence platitudinarian (n.) "one who indulges in platitudes," 1855; platitudinize (1867).
A commonplace is a thing that, whether true or false, is so regularly said on certain occasions that the repeater of it can expect no credit for originality ; but the commonplace may be useful.
A platitude is a thing the stating of which as though it were enlightening or needing to be stated convicts the speaker of dullness ; a platitude is never valuable. [Fowler]
1816, ornary, American English dialectal contraction of ordinary (adj.). "Commonplace," hence "of poor quality, coarse, ugly." By c. 1860 the sense had evolved to "mean, cantankerous." Related: Orneriness.