Rest assured, (sentence)

This phrase means something like "Don't worry". You say this when you're confident that something will happen. You're making a sort of promise that you will accomplish something:

Rest assured, we will not stop until we have recovered every single one of the hostages.

This expression sounds very strong and determined. It's too strong to use in common, everyday situations.

(someone is) a scumbag

A "scumbag" is a really horrible person. You might call someone a "scumbag" if they do something like:

• kidnap a child
• abuse their wife or girlfriend
• steal millions of dollars from the company they work for

The word "scumbag" is usually applied to adult men, not to women or children.

make (someone) pay

To "make someone pay" means to punish them or get revenge.

You can use "for" to show why you want to get revenge on the person. For example, if someone insults you, you might think:

I'll make you pay for saying that!