have a big mouth字面意思是有一张大嘴巴，即多嘴；口无遮拦；嘴巴不严。

【例句】
I'm sorry, I shouldn't have opened my big mouth.
对不起，我不应该口无遮拦。

If I'd known you had such a big mouth, I'd have never shared my secret with you!
如果我早知道你嘴巴不严，我就不会和你分享我的秘密了。