na-adjective, noun
cruel; hard-hearted; harsh; unkind
Describes a cold, unfeeling attitude or treatment toward others. Often used in the phrase 邪険にする (to treat harshly/coldly).
彼は困っている友人を邪険に扱った。
He treated his troubled friend harshly.
そんな邪険な態度はよくない。
Such a cruel attitude is not good.
The kanji 邪 (wicked) and 険 (steep, harsh) combine to suggest a harsh, unkind nature. The exact historical derivation is uncertain.