所為
せい · sei · JLPT N3 · noun
cause
せい sounds like say: say whose fault it is, say the cause
所為 is ateji: 所 place + 為 doing, so the place where the deed came from. It is usually written in kana, as in 君のせいだ, it is your fault. It often carries blame. Use おかげ when the result was good.
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.