Etymology
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depression (n.)

c. 1400 as a term in astronomy, "angular distance of a star below the horizon," from Old French depression (14c.) and directly from Medieval Latin depressionem (nominative depressio), noun of action from past participle stem of Latin deprimere "to press down, depress" (see depress).

The literal sense "act of pressing down, state of being pressed down" is attested from 1650s. The meaning "dejection, state of sadness, a sinking of the spirits" is from early 15c. (as a clinical term in psychology, from 1905); meteorological sense is from 1881 (in reference to barometric pressure); meaning "a lowering or reduction in economic activity" was in use by 1826; given a specific application (with capital D-) by 1934 to the one that began worldwide in 1929. For "melancholy, depression" an Old English word was grevoushede.

A melancholy leading to desperation, and known to theologians under the name of 'acedia,' was not uncommon in monasteries, and most of the recorded instances of medieval suicides in Catholicism were by monks. [Lecky, "History of European Morals"]
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groin (n.)
"oblique depression of the body between the abdomen and thighs," 1590s, earlier grine (1530s), from Middle English grynde "groin" (c. 1400), originally "depression in the ground," from Old English grynde "abyss," perhaps also "depression, hollow," from Proto-Germanic *grundus (see ground (n.)). Altered 16c. by influence of loin or obsolete groin "snout of a pig." The architectural groin "curving edge formed by the intersection of two vaults" is from 1725.
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hypo (n.)
1711, "depression of the spirits," short for hypochondria; 1904 as short for hypodermic needle.
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blues (n.2)
"depression, low spirits," 1741, from blue (adj.1) in the sense "low-spirited" (c. 1400).
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indentation (n.2)
"action of making a dent or impression; small hollow or depression, slight pit," 1847, from indent (v.2).
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pit-viper (n.)

venomous serpent, so called from the characteristic depression between the eyes and the nose, by 1872, from pit (n.1) + viper.

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melancholia (n.)

"mental condition characterized by great depression, sluggishness, and aversion to mental action," 1690s, from Modern Latin melancholia (see melancholy).

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cupule (n.)

"small cup-shaped depression or object," 1830, from Modern Latin cupula, diminutive of Latin cupa "cask, barrel" (see cup (n.)).  

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geosyncline (n.)
1895, probably a back-formation from adjective geosynclinal (1879); see geo- + synclinal. Geosynclinal was used as a noun meaning "a region of depression" from 1873.
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blah (n.)
"idle, meaningless talk," 1918, probably echoic; the adjective meaning "bland, dull" is from 1919, perhaps influenced by French blasé "bored, indifferent." The blahs "depression" is attested by 1966.
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