在庫
ざいこ · zaiko · JLPT N1 · noun
reserve
📦 在 (zai, 'at/in') + 庫 (ko, 'warehouse') = goods kept at the warehouse, i.e. stock on hand
在庫 literally means 'present in the storehouse.' 在 is 'be located at' and 庫 is 'warehouse,' so the word names goods sitting in storage, ready to sell.
在 — earth 土 + phonetic 才
Originally a phono-semantic compound: 土 (earth) gives the meaning of a place or location, while 才 supplies the sound. It came to mean 'to be present/located at,' and now also serves as a grammatical marker for ongoing actions, like 'in the middle of doing.'