shoot off原意是发射子弹、炮弹。所以shoot off one's mouth按字面意思就是从咀里里发射炮弹、子弹，即滔滔不绝；像连珠炮似地讲；信口开河；瞎说乱讲。

【例句】
He's always shooting off his mouth about things he knows nothing about and I can't stand that loud voice.
他老是对自己一无所知的事情滔滔不绝，而且他也受不了Harry那大嗓门。

If you shoot your mouth off like that one more time, I won't tell you anything in the future.
要是你下次再这么胡说八道，以后我什么也不告诉你了。