Depicts a shield. Based on the original meaning "firm, solid", which is now written as 固. The current meaning "old" is a phonetic loan.
Shuowen Jiezi
《說文》：“古，故也。从十口。識前言者也。，古文古。”
Original meaning
firm, solid
Components
◎(iconic) — Depicts a shield.
口(meaning) — The 口 component is sometimes used to indicate an abstract meaning taken from a concrete object. Here, the concrete object "shield" evokes the abstract meaning "…