The word "一下子" is formed through the compounding and affixation of basic linguistic components: the numeral "一" (one), the directional character "下" (down or a single instance), and the common nominal suffix "子". The core idea originates from the concept of "a single downward movement" or "one go", which metaphorically evolved to signify an action happening in one brief, immediate, and often sudden unit of time. The suffix "子" is added to solidify this phrase into a fixed, standalone adverb, cementing its meaning of "all at once" or "in a flash".