early 14c., "fail in strength, exhibit signs of approaching death," from languiss-, present participle stem of Old French languir "be listless, pine, grieve, fall ill" (12c.), from Vulgar Latin *languire, from Latin languere "be weak or faint" (from PIE root *sleg- "be slack, be languid"). Weaker sense of "be lovesick, grieve, lament, grow faint," is from mid-14c. Related: Languished; languishing.
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Harper, D. (n.d.). Etymology of languishing. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/languishing