"on the fly" (no hyphens) is an idiom used as an adverb, meaning "in midair" or "as something is happening". It does suggest speed abd agility. It comes from baseball, meaning to catch the ball before it hits the ground.
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AlpheccaStars "on the fly" (no hyphens) is an idiom used as an adverb...But is this still some kind of compound? Perhaps, an idiomatic compound adverb?