na-adjective, noun
doing as one pleases; following one's whims
Describes acting on impulse or personal inclination without a fixed plan. Often used in the phrase 気任せに.
今日の予定は気任せに決めよう。
Let's decide today's plans on a whim.
彼の行動はいつも気任せだ。
His actions are always just as he pleases.
Compound of 気 (ki, 'mind, spirit') and 任せ (makase, 'leaving to'), from the verb 任せる (makaseru, 'to entrust'). Literally 'leaving it to one's mood'.