Two separate meanings: (1) the historical-linguistic sense —
中古汉语 = Middle Chinese, the language of the Sui-Tang reconstructed from rhyme books like 《
切韵》; (2) a Japanese loan
中古 (chūko) = 'used/second-hand'. Young Chinese speakers increasingly use
中古 in sense (2) for second-hand luxury goods (
中古包,
中古店).