noun, na-adjective
self-willed; doing as one pleases
Describes acting entirely according to one's own desires or convenience, often with a negative connotation of selfishness or disregard for others. Combines 気まま (selfish, willful) and 勝手 (selfish, one's own way) for emphasis.
彼の気まま勝手な行動に周りは振り回されている。
Everyone around him is being dragged around by his self-willed behavior.
気まま勝手に振る舞うのはやめてほしい。
I wish you would stop acting so selfishly.
気まま alone means 'selfish' or 'willful' but can also have a lighter nuance of 'carefree' or 'doing as one likes' without strong negative judgment. 気まま勝手 is more emphatically negative.
勝手 alone means 'one's own convenience' or 'selfishness'. 気まま勝手 combines both for a stronger expression of self-centered behavior.
Compound of 気まま (selfish, willful) and 勝手 (selfish, one's own way), both native Japanese words. The exact historical derivation is uncertain, but the combination reinforces the meaning of acting solely for one's own convenience.