The word "一口气" is formed by sequentially combining the character "一" meaning "one", "口" meaning "mouth", and "气" meaning "breath" or "air", resulting in a literal phrase "one mouth breath". This literal composition conveys the concept of a single inhalation or exhalation, which is used metaphorically to denote completing an action in one continuous, uninterrupted effort. Each character retains its individual meaning to contribute directly to this aggregated sense.