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?
大きい犬、平気ですか？
Are you okay with large dogs?
このホテル、火が出てるけど平気？
There's fire coming out of this hotel, is everything okay?