be tested time and again and proved fight; it has proved effective (/it comes out right) every time; put to repeated tests and proved satisfactory
❍ 这种新杀虫药效果良好，～。This new insecticide has proved effective every time it is used.
屡试不爽lǚ shì bù shuǎnɡ
爽：差错。多次试验都没有差错。time-tested, it comes out right every time, put to repeated tests and prove right