fast-acting

The compound adjective "fast-acting" is a modern formation that logically combines two established English elements. The first element, "fast", originates from a Germanic root meaning "firm" or "secure", a sense which evolved in Middle English to denote "rapid", likely from the concept of being steadfast in movement or running. The second element, "acting", derives from the Latin verb agere, meaning "to drive, lead, or do", which is the source of a vast family of English words related to action and agency. The compound directly and transparently describes a substance, typically a medication or chemical, that performs its intended function with speed, linking the ancient concept of vigorous action with the modern expectation of rapid efficacy.

Structure: fast (quick) + acting (performing an action) - fast: From Old English fæst meaning "firmly fixed, steadfast, secure". In this compound, it adopts the later sense of "quick, rapid". - acting: From Latin agere (to do, drive, act), via the present participle form act- and the English suffix -ing, forming an adjective meaning "performing an action".