verb, adverb · without authorization or knowledge of others; privately; secretly.

他竟敢私自拿走公司的重要文件，真是太胆大了。

noun, idiom · relating to an individual's personal affairs rather than public or official matters

他这次出国是为了处理一些私事。

noun · motivated by one's own interests or seeking personal gain

他一心只顾私利，不顾集体利益。