“He said it so matter-of-factly, I didn't know what to make of it at first.”

The guy you've been dating told you that he's moving away to another country. You're shocked because you thought the relationship was serious, but he didn't seem to care. Now you're telling a friend the story of what happened. You say:

He said it so matter-of-factly, I didn't know what to make of it at first.

(say something) matter-of-factly

Saying something "matter-of-factly" means that you say it without much emotion, the way that you would say a simple sentence like "It's five o'clock".

(someone) didn't know what to make of (something)

This phrase means that a person didn't understand something or didn't know what it was. In the example above, the speaker didn't know whether the guy was serious or joking, whether he really didn't care about her or whether he was trying to hide his true feelings.

Other feelings that you might have when you "don't know what to make of" something are:

• You're not sure whether something is good or bad.
• You can't decide what something is.
• You don't know why a person said something the way that they said it.

at first

Use "at first" to describe something that happens at the beginning of a period of time:

At first I thought he was joking. Then I realized that he was serious.

There weren't many people using it at first. But it gradually started to catch on.

"At first" is different from "first", which you use for listing things in order:

First you chop up all the vegetables. Then you heat the pan on medium-high heat and add a bit of oil...