go off at half cock表示操之过急；操之过急，因为冒失而失败。

【例句】
Jane was thinking about quitting her present job, but I advised her not to go off at half cock.
简打算辞去现职，但我劝她不要操之过急。

His schemes always go off at half cock because he never prepares them properly.
他的计划总是仓促而行，因为他从不好好准备。