noun
malicious intent; inclination for wrongdoing
Refers to a person's inner disposition toward evil or criminal acts. Often used in literary or formal contexts to describe a corrupt or treacherous heart.
彼の行動には賊心が感じられた。
His actions betrayed a malicious intent.
noun
rebellious mind
Specifically denotes a mindset of rebellion or defiance against authority. Less common than the general sense of malicious intent.
賊心を抱く者たちが反乱を企てた。
Those harboring rebellious minds plotted an uprising.
Compound of 賊 (thief, rebel) and 心 (heart, mind). The exact historical derivation is uncertain, but the combination naturally yields the meaning of a thievish or rebellious heart.