Etymology
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sadness (n.)

early 14c., sadnesse, "seriousness," from sad + -ness. Meaning "sorrowfulness, dejection of mind" is by c. 1500, perhaps c. 1400, but throughout Middle English the word usually referred to "solidness, firmness, thickness, toughness; permanence, continuance; maturity; sanity."

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lugubriosity (n.)

"sorrowfulness, sadness," 1839, abstract noun from lugubrious. Sometimes also lugubrosity.

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melancholy (adj.)

late 14c., malencolie, "mixed with or caused by black bile;" also, of persons, "sullen, gloomy, sad, affected by low spirits," from melancholy (n.). Meaning "expressive of sadness" is from 1590s; sense of "deplorable, fitted to produce sadness or gloom" (of a fact or state of things) is by 1710.

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heaviness (n.)
Old English hefigness "state of being heavy, weight; burden, affliction; dullness, torpor;" see heavy (adj.) + -ness. Chaucer has heavity for "sadness."
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trist (adj.)
"sorrowful," early 15c., from French triste "sad, sadness" (10c.), from Latin tristis "sad, mournful, sorrowful, gloomy." Re-borrowed late 18c. (as "dull, uninteresting") as a French word in English and often spelled triste.
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longing (n.)

"yearning, eager desire, craving," Old English langung "longing, weariness, sadness, dejection," verbal noun from long (v.).

2. Specifically, in pathol., one of the peculiar and often whimsical desires experienced by pregnant women. [Century Dictionary, 1899]
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rue (n.2)

"sorrow, repentance," Middle English reue, from Old English hreow "grief, repentance, sorrow, regret, penitence," from Proto-Germanic *hrewwo "pain; sadness, regret, repentance," source also of Frisian rou, Middle Dutch rou, Dutch rouw, Old High German (h)riuwa, German reue "sorrow, regret, repentance," nouns from the root of rue (v.).

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mourning (n.)

"feeling or expression of sorrow, sadness, or grief," c. 1200, from Old English murnung "complaint, grief, act of lamenting," verbal noun from mourn (v.). Meaning "customary dress or garment worn by mourners" is from 1650s (mourning habit is from late 14c.). The North American mourning dove (1820) is so called for its call.

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lugubrious (adj.)

"expressing or characterized by sadness or mournfulness; doleful," c. 1600, formerly also lugubrous, from -ous + Latin lugubris "mournful, doleful, pertaining to mourning," from lugere "to mourn," from PIE root *leug- "to break; to cause pain" (source also of Greek lygros "mournful, sad," Sanskrit rujati "breaks, torments," Lettish lauzit "to break the heart"). Related: Lugubriously; lugubriousness.

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pensive (adj.)

late 14c., pensif, "sad, sorrowful, melancholy;" also "engaged in serious thought, meditative, contemplative;" from Old French pensif "thoughtful, distracted, musing" (11c.), from penser "to think," from Latin pensare "weigh, consider," frequentative of pendere "to hang, cause to hang; weigh; pay" (from PIE root *(s)pen- "to draw, stretch, spin"). Meaning "expressing thoughtfulness with sadness" is from 1540s. Related: Pensively; pensiveness.

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