evil heart; wicked mind
A somewhat literary or emphatic expression for a fundamentally corrupt or malicious inner nature. Often used in moral or religious contexts.
Behind his actions lurked an evil heart.
Compound of 邪ま (よこしま, 'wicked, unjust') + な (attributive) + 心 (こころ, 'heart, mind'). The adjective 邪ま is of native origin, related to 横 (よこ, 'side, wrong direction').