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1610s, from French flaccide or directly from Latin flaccidus "flabby," from flaccus "flabby, flap-eared," of uncertain origin (OED suggests it's imitative). Related: Flaccidly; flaccidity.
flaccid flac·cid (flāk'sĭd, flās'ĭd)
Lacking firmness, resilience, or muscle tone.