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.
He always has a smug look on his face.
Don't get carried away just because you're feeling good about yourself.
図に乗る means 'to get carried away' or 'to push one's luck', similar to the negative sense of いい気になる but more about taking advantage of a situation.
serves you right; you deserve it
Most common spelling; uses the hiragana いい for the adjective stem.
Kanji spelling using 好い; less common but still encountered.
Kanji spelling using 良い; rare and may be confused with よいき.
Compound of いい (good) and 気 (spirit/mood). The meaning 'conceited' likely developed from the idea of being in a self-satisfied mood.