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.