这个习语来自于莎士比亚的《亨利六世》。Seal up your lips and give no words but mum.直接翻译是：闭上嘴巴，啥也不说，除了保持沉默。于是，mum is the word就从句子中提取出来，用在提醒别人保持沉默，恪守诺言的语境。
其中的"mum"是"momme"的俚语说法，在中古英语中是“保持沉默”的意思。"mum"的这层意思，是由"murmur"--“哑剧演员”发展而来。
A boy presents a handkerchief he made for his mother at a kindergarten in Hefei, capital of East China's Anhui province, May 9, 2013. Children made cards, handkerchiefs and other gifts for their mother to celebrate the coming Mother's Day. [Photo/Xinhua]举个例子：
“I won the lottery!”Paul whispered in his wife's ear.
“我中彩了!”保罗在妻子的耳边悄声说。
“Don't breathe a word!”
“千万要保密!”
“Okay，honey;mum is the word.”
“好的，绝对不讲出去。”
那么，当别人跟你说Mum's the word让你给他保密时，除了Yes还可以怎么回答得更加真诚呢?
I won't say a word.
我保证对谁都不说出去。
I won't say anything.
我什么都不说。
My lips are sealed.
我的嘴很紧。
当然可能有些大嘴巴，忍不住就说出去了，通常开场白是这样的：我跟你说件事，你不要告诉别人哦!然后就开始泄露秘密了。
I'll level with you.
好吧，我就直说吧。
I have a confession to make.
有件事我一定要坦白。
Well, as a matter of fact...
好吧，说实话……
那如果是你不小心说漏嘴了咋办?
It was a slip of the tongue.
无意中说漏了嘴。
Really? Didn't she know about it?
真的吗?她不知道这件事?
I spilled the beans.
我泄露了秘密。
I stuck my foot in my mouth.
我把不好/失礼的事说出去了。
不过做人嘛，还是要有分寸和底线的。当你忍不住要说出去的时候，就想一想：别人跟你说这个秘密的时候，有多信任你吧!
另外，我们整理了一份和妈妈有关的表达。
01 Mummy's boy
Mummy's boy指的是：很听妈妈的话，什么事情都让母亲来做主，而自己毫无主见的男生或者成年男性。也就是我们常说的“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。
Tim is a mummy's boy. He left his girlfriend because his mother didn't like her.
提姆是一个妈宝男，因为他妈妈不喜欢他的女朋友，所以他离开了对方。
02 everyone and their mum
Everyone and their mum:
每个人和他们的妈妈 ×
大多数人，很多人 √
这个短语，用夸张的手法来形容“非常多的人，几乎每一个人”
Everyone and their mum is going on a vacation, but I have to work.
大家都要休假了，可我还得工作。
03 stay-at-home mom
Stay-at-home mom:大家应该很好猜出来是什么意思吧?呆在家里不外出工作，全身心在家带宝宝的妈妈，也就是“全职妈妈”。
Being a stay-at-home mom is as much hard work as going to an office everyday.
全职妈妈和每天出去工作一样辛苦。
John: So we've been talking about women's roles in society. And are you a stay-at-home mom or a working mom?
约翰：我们来谈谈女性在社会中的角色。你是全职妈妈还是职场妈妈?
Sarah: Well, I guess I'm both. Right now, I'm on maternity leave and I'm on six months of maternity leave.
萨拉：嗯，我想我两者都是。目前我在休产假，我正在休为期6个月的产假。
John: What does that mean?
约翰：这是什么意思?
Sarah: So maternity leave is when you have a job but then you become pregnant or you're going to have a baby, maybe adopt, too. So you're going to have a baby, and so you take time off of work to stay home with the new baby.
萨拉：产假是指你有工作，不过因为你怀孕了或者即将生产，或是领养，在你要生产的时候，你可以休假，留在家里照顾孩子。
04 mother's knee
At your mother's knee直译过来是“在你妈妈的膝上”，而实际要表达的是：幼年时或是儿时学到的技能，比如画画，钢琴，舞蹈这些启蒙技能。
举个例子，小白很小的时候就学会了画画，他就可以这样说：
I learnt to paint at my mother's knee.
我在幼年时学会了涂色。
