bô-tī-bô-tāi無底無代

without cause or reason

without cause or reason; for no reason at all

English Wiktionary, “無底無代” (CC BY-SA 4.0) — https://en.wiktionary.org/wiki/%E7%84%A1%E5%BA%95%E7%84%A1%E4%BB%A3 · confidence: Medium (Wiktionary)