i-adjective
casual; unconcerned; without thinking
Describes an action or attitude done without deep thought, worry, or ulterior motive; often used in the adverbial form 何心なく.
See also: 何心無く
何心なく窓の外を見ていた。
I was casually looking out the window without any particular thought.
彼は何心ない様子で質問に答えた。
He answered the question with an unconcerned air.
何気ない is more common and means 'casual' or 'nonchalant', often implying an attempt to appear indifferent. 何心ない is less frequent and emphasizes a genuine lack of concern or thought.
Compound of 何 (nani, 'what') + 心 (kokoro, 'heart/mind') + ない (nai, 'not'). The exact historical derivation is uncertain, but the word literally suggests 'without any particular heart/mind'.