c. 1600, "make Roman in character," from Roman + -ize. Intransitive sense of "follow Roman customs" is by 1620s; that of "become a Roman Catholic" is by 1630s. Related: Romanized; Romanizing; Romanization.
noun and adjective, Old English, "of or pertaining to ancient Rome; an inhabitant or native of ancient Rome," from Latin Romanus "of Rome, Roman," from Roma "Rome" (see Rome). The adjective is c. 1300, from Old French Romain. The Old English adjective was romanisc, which yielded Middle English Romanisshe.
In reference to a type of numeral (usually contrasted to Arabic) it is attested from 1728; as a type of lettering (based on the upright style typical of Roman inscriptions, contrasted to Gothic, or black letter, and italic) it is recorded from 1510s. The Roman nose, having a prominent upper part, is so called by 1620s. The Roman candle as a type of fireworks is recorded from 1834. Roman Catholic is attested from c. 1600, a conciliatory formation from the time of the Spanish Match, replacing Romanist, Romish which by that time had the taint of insult in Protestant England.
word-forming element used to make verbs, Middle English -isen, from Old French -iser/-izer, from Late Latin -izare, from Greek -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached.
The variation of -ize and -ise began in Old French and Middle English, perhaps aided by a few words (such as surprise, see below) where the ending is French or Latin, not Greek. With the classical revival, English partially reverted to the correct Greek -z- spelling from late 16c. But the 1694 edition of the authoritative French Academy dictionary standardized the spellings as -s-, which influenced English.
In Britain, despite the opposition to it (at least formerly) of OED, Encyclopaedia Britannica, the Times of London, and Fowler, -ise remains dominant. Fowler thinks this is to avoid the difficulty of remembering the short list of common words not from Greek which must be spelled with an -s- (such as advertise, devise, surprise). American English has always favored -ize. The spelling variation involves about 200 English verbs.