For a very short time; passing quickly; in a brief, transitory manner.
From 'fleeting' (adjective, from 'fleet' plus '-ing'), plus the adverbial suffix '-ly,' creating an adverb that describes how briefly or quickly something happens or exists.
We use 'fleetingly' constantly in modern prose because it captures something specific—a thought or expression that passes so quickly it barely registers, like a ship glimpsed then lost to fog.
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