古代
こだい · kodai · JLPT N1 · noun
times
古 — ten 十 + mouth 口
古 is traditionally analyzed as 十 (ten) over 口 (mouth), suggesting words passed down through ten generations — hence 'ancient' or 'old.' Some scholars view the top as a stylized helmet or skull shape in oracle bone forms. The core meaning of 'ancient, old, former times' has remained stable since early Chinese writing.
代 — person radical 亻 + 弋 (stake/dart, phonetic)
代 is a semantic-phonetic compound: 亻 (person) gives the meaning and 弋 supplies the sound. The original sense was one person taking the place of another, which broadened to 'substitute,' 'replace,' and then 'generation' or 'era' — successive groups of people replacing the previous.