go off就是"开火"的意思，go off half-cocked从字面上看来是枪的保险扳机还没扳上就发火了，而当时的步枪由于开火装置上的缺陷，时常会出这样的毛病。即发脾气；轻率作出反应。

【例句】
Now, don't go off half-cocked and do something foolish.
喂！不要轻率地行动做出什么傻事来。

She wanted to quit her job but her mother told her not to go off half - cocked.
她想离职,但她妈妈告诉她不要仓促行事。