Mum's the word
keep mum
keep your mouth shut
between you and me
保密噢～

everyone and their mom
(夸张地讲)大部分人，绝大多数人
Everyone and their mom seems to be going to concert tonight.
所有人(夸张)都去了那个演唱会。

mummy's boy
妈宝男
Don't marry him! He's just a mummy's boy.
别嫁给他！他就是一个妈宝男。

Mother knows best.
母亲说得最有道理！妈妈知道什么对你最好！妈妈最清楚！母亲最了解！
stay-at-home mom
全职妈妈
mom brain
当妈的脑子，一孕傻三年