Bottom mouth — the indexing radical. Speech is the medium that carried antiquity across generations: stories, laws, poems retold mouth to mouth. 口 anchors 古 in the speech-and-tradition family alongside 言 (word), 史 (history), 名 (name).
Top ten — a numeric stand-in for 'many.' The whole character reads as 'ten generations of mouths' — stories passed from mouth to mouth across ten lifetimes. That orally-transmitted weight gives the meaning of ancient, time-tested, classical.
