Describes a person or behavior that is cunning and deceitful in a malicious way, often hiding true intentions behind a pleasant facade.
He used underhand tactics to bring down his rival.
That person thinks sly things behind a smile.
Specifically refers to a facial expression or appearance that seems sly, malicious, or untrustworthy. Often used with 顔, 表情, or 目つき.
His sinister look bothered me.
陰険 means sly and malicious in a scheming way, whereas 陰気 simply means gloomy or cheerless without implying any hidden malice.
Standard kanji spelling.
Kana spelling is sometimes used in casual writing or when the kanji are considered difficult.
Compound of 陰 (in, 'shadow, hidden') and 険 (ken, 'steep, severe, dangerous'), suggesting a hidden dangerous or malicious nature.