The beginning of something, especially something unpleasant or noticeable.
From "on" and "set," originally meaning an attack or assault that is "set on" an enemy. Over time it broadened to mean the start of any event or condition.
The original sense of "attack" still echoes when doctors talk about the "onset" of a disease—it’s like the illness is launching an assault. The word turns a simple beginning into something active and forceful.
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