快马加鞭 is formed from the characters 快 (fast), 马 (horse), 加 (add), and 鞭 (whip), which are combined in sequence to literally mean "to add a whip to a fast horse". This direct juxtaposition creates a vivid image of whipping a horse that is already swift, and the idiom thus metaphorically signifies hastening or accelerating something that is already progressing rapidly.