Moving or able to move quickly; happening in a short time. As a verb, it also means to go without food for a period of time, especially for religious or health reasons.
The “quick” sense comes from Old English “fæst” meaning “firm, fixed,” which developed into “firmly, strongly” and then “quickly.” The “not eating” sense comes from Old English “fæstan” meaning “to observe abstinence,” possibly related but with a different development.
Strangely, “fast” once meant “firm” or “fixed,” which is why we still say “hold fast” or “stuck fast.” The speed meaning grew from the idea of doing something firmly and intensely—language flipped from unmoving to very quickly moving.
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