Calm
Cool, All Right, Okay
noun, な adjective
You have a flat energy. What kind of energy would be a flat one? Probably a cool or calm one. You'd also tell people about this flat energy by saying you are all right or just okay.
へいき
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
よく平気でいられますね。
How come you’re always so calm?
王女はペットの子犬をなくしても平気な人です。
The princess is a type of person who wouldn't get upset even if she lost her pet puppy.
夫が会社を首になったが、意外と平気だ。
My husband was fired, but I’m surprisingly calm.