bô-in-bô-toaⁿ無因無端

without cause or reason

without cause or reason; for no reason at all

English Wiktionary, “無因無端” (CC BY-SA 4.0) — https://en.wiktionary.org/wiki/%E7%84%A1%E5%9B%A0%E7%84%A1%E7%AB%AF · confidence: Medium (Wiktionary)