like a thief in the nightadvadverb: Describes a verb, adjective, adverb, or clause--for example, "come quickly," "very rare," "happening now," "fall down."
(silently, stealthily)
偷偷摸摸地 tōu tōu mō mō de
The fog crept down the valley like a thief in the night. The cat crept through the garden like a thief in the night.