na-adjective
Describes a person or behavior that is cunning and deceitful in a malicious way, often hiding true intentions behind a pleasant facade.
彼は陰険な手口でライバルを蹴落とした。
He used underhand tactics to bring down his rival.
あの人は笑顔の裏で陰険なことを考える
That person thinks sly things behind a smile.
na-adjective
Specifically refers to a facial expression or appearance that seems sly, malicious, or untrustworthy. Often used with 顔, 表情, or 目つき.
彼の陰険な目つきが気になった。
His sinister look bothered me.
Compound of 陰 (in, 'shadow, hidden') and 険 (ken, 'steep, severe, dangerous'), suggesting a hidden dangerous or malicious nature.