noun-modifying (irregular)
Old attributive form of 古い (ふるい). Used prenominally in classical or literary contexts, often in fixed expressions like 古き良き時代 (the good old days). Not used in modern conversational Japanese.
古き良き時代を懐かしむ。
I feel nostalgic for the good old days.
この町には古き伝統が残っている。
Old traditions remain in this town.
Archaic attributive form of the adjective 古い (ふるい). The -ki ending is the classical Japanese attributive form for -i adjectives.