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1610s, "of the highest class; approved as a model," from French classique (17c.), from Latin classicus "relating to the (highest) classes of the Roman people," hence, "superior," from classis (see class). Originally in English, "of the first class;" meaning "belonging to standard authors of Greek and Roman antiquity" is attested from 1620s.
"a Greek or Roman writer or work," 1711, from classic (adj.). So, by mid-19c., any work in any context held to have a similar quality or relationship. In classical Latin noun use of classicus meant "a Marine" (miles classicus) from the "military division" sense of classis.