c. 1400, ordinarie, "regular, customary, belonging to the usual order or course, conformed to a regulated sequence or arrangement," from Old French ordinarie "ordinary, usual" and directly from Latin ordinarius "customary, regular, usual, orderly," from ordo (genitive ordinis) "row, rank, series, arrangement" (see order (n.)).
From 1580s as "common in occurrence, not distinguished in any way." Its various noun uses, dating to late 14c. and common until 19c., are now largely extinct except in out of the ordinary (1893) in which the sense of ordinary is "established or due sequence; something regular or customary." In British education, Ordinary level (abbrev. O level), "lowest of the three levels of General Certificate of Education," is attested from 1947. Related: Ordinarily.
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.
It forms all or part of: adorn; alarm; aristarchy; aristo-; aristocracy; arm (n.1) "upper limb of the body;" arm (n.2) "weapon;" armada; armadillo; armament; armature; armilla; armistice; armoire; armor; armory; army; art (n.) "skill as a result of learning or practice;" arthralgia; arthritis; arthro-; arthropod; arthroscopy; article; articulate; artifact; artifice; artisan; artist; coordination; disarm; gendarme; harmony; inert; inertia; inordinate; ordain; order; ordinal; ordinance; ordinary; ordinate; ordnance; ornament; ornate; primordial; subordinate; suborn.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit irmah "arm," rtih "manner, mode;" Armenian arnam "make," armukn "elbow;" Greek arti "just," artios "complete, suitable," artizein "to prepare," arthron "a joint;" Latin ars (stem art-) "art, skill, craft," armus "shoulder," artus "joint," arma "weapons;" Old Prussian irmo "arm;" German art "manner, mode."
"ordinary dress of civil life" (as opposed to military uniform), 1822; in reference to police detectives, it is attested from 1842. Also plainclothes.
early 15c., cors "ordinary" (modern spelling is from late 16c.), probably adjectival use of noun cours (see course (n.)). Originally referring to rough cloth for ordinary wear, the sense of "rude, vulgar, unpolished" developed by c. 1500 and that of "obscene" by 1711.
Perhaps via the notion of "in regular or natural order," hence "common, vulgar" (compare the development of mean (adj.), also ornery from ordinary). Or it might be via the clothing sense, and the notion of "wanting fineness of texture or elegance of form." Or both, and there might be also an influence, via metathesis, of French gros (see gross (adj.)), which underwent a similar sense development. Related: Coarsely; coarseness.