na-adjective, noun
easygoing; optimistic; conceited
Describes a person who is carefree, overly optimistic, or smugly self-satisfied. Often used in the phrase いい気になる (to become conceited or get a swelled head). The nuance can range from positive (relaxed) to negative (smug) depending on context.
彼はいつもいい気な顔をしている。
He always has a smug look on his face.
いい気になって調子に乗るな。
Don't get carried away just because you're feeling good about yourself.
Compound of いい (good) and 気 (spirit/mood). The meaning 'conceited' likely developed from the idea of being in a self-satisfied mood.