平白無故

平白无故
píngbáiwúgù
(idiom) for no reason whatever; inexplicably

CEDICT

without any cause or reason; groundless

Wiktionary

無緣無故。

重編國語辭典

Entry Composition

1 平白 píngbái
  1. for no reason
  2. gratuitously
2 無故 wúgù without cause or reason