The idiom "毁于一旦" is formed by the sequential combination of four characters: 毁 (huǐ), meaning "to destroy"; 于 (yú), a preposition meaning "in" or "at"; 一 (yī), meaning "one"; and 旦 (dàn), meaning "dawn" or "daybreak". Together, they literally translate to "destroyed in one dawn", with the characters' individual meanings directly constructing the figurative sense of something being ruined completely and abruptly within a very short timeframe.