Mum's the word

The expression "mum's the word" is used to say that you have promised not to tell anyone someone's secret.

For example, if you are passing a secret along, you can use this expression:
A: We're having a surprise birthday party for Tyler on Saturday.
B: Awesome.
A: Mum's the word, okay?
B: Got it.

The "mum" in this phrase isn't mother. It's a Middle English word meaning "silent".

It's raining cats and dogs

The expression means that it's raining very heavily. It's an interesting old English phrase that no one knows the origin of it.

When it is raining very heavily, we may also use the following idiomatic expressions:
• It's pouring down
• It's pouring from the heavens
• It's bucketing down
• It's flooding down
• It's pissing down
• It's peeing down very colloquial
Improved by Kathy Giles

to feel under the weather

The expression means to not feel very well, or be tired or stressed.
I'm not going to work today as I'm feeling a bit under the weather.

At the drop of a hat

To do something at the drop of a hat means to do it instantly, without any delay.
If you need any help, just phone me. I'll be there at the drop of a hat.

The idiom may have come from the American Old West; in the 19th century it was occasionally the practice to signal the start of a fight or a race by dropping a hat or sweeping it downward while holding it in the hand.

cover for

To hide someone's wrongdoing from someone else, by lying for them.
If I go to the party tonight, will you cover for me? Just tell mom that we're studying for the exam.

Temporarily, doing the work in place that someone else usually does, because they are not there.
Joanna, will you cover for me for one hour, I gotta go to the kid's school.