noun
rebellious spirit; spirit of revolt
Refers to a mindset or feeling of defiance against authority or the status quo. Often used in literary or formal contexts.
彼の言動には反心が感じられた。
A rebellious spirit could be felt in his words and actions.
反抗心 is a more common and everyday word for a rebellious or defiant attitude, while 反心 is more literary and abstract.
Compound of 反 (anti-, against) and 心 (heart, spirit). The variant 叛心 uses 叛 (rebel). The exact historical derivation is uncertain, but the meaning is transparent from the components.