所
suǒ · HSK 3
that which
户 (door) + 斤 (axe) — an axe by the door marks the place where
所 originally depicted the sound of chopping wood (斤 axe) near a door (户), later borrowed for 'place' and the grammatical 'that which'. In modern use it nominalises verbs: 所说 = that which is said.
所 — door 戶 (left) + axe 斤 (right)
Originally a sound: an axe (斤) chopping wood near a door (戶), with 戶 also hinting at the pronunciation. Over time it was borrowed to mean 'place' or 'location,' and later became a grammatical particle nominalizing verbs ('that which…'). The concrete 'chopping' sense faded entirely.