Etymology
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natural (n.)

"person with a natural gift or talent," 1925, originally in prizefighting, from natural (adj.). But an older sense is almost opposite to this, "half-wit, idiot" (one "naturally deficient" in intellect), which was in use 16c. to 19c. In Middle English, the word as a noun meant "natural capacity, physical ability or power" (early 14c.), and it was common in sense "a native of a place" in Shakespeare's day. Also in 17c., "a mistress."

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natural (adj.)

c. 1300, naturel, "of one's inborn character; hereditary, innate, by birth or as if by birth;" early 14c. "of the world of nature (especially as opposed to man)," from Old French naturel "of nature, conforming to nature; by birth," and directly from Latin naturalis "by birth, according to nature," from natura "nature" (see nature).

Of events, features, etc., "existing in nature as a result of natural forces" (that is, not caused by accident, human agency, or divine intervention), late 14c. From late 14c. of properties, traits, qualities, "proper, suitable, appropriate to character or constitution;"  from late 15c. as "native, native-born." Also late 15c. as "not miraculous, in conformity with nature," hence "easy, free from affectation" (c. 1600). Of objects or substances, "not artificially cultivated or created, existing in nature" c. 1400. As a euphemism for "illegitimate, bastard" (of children), it is recorded from c. 1400, on the notion of blood kinship (but not legal status).

Natural science, that pertaining to physical nature, is from late 14c.;  natural history meaning more or less the same thing is from 1560s (see history).  Natural law "the expression of right reason or the dictate of religion inhering in nature and man and having ethically binding force as a rule of civil conduct" is from late 14c. Natural order "apparent order in nature" is from 1690s. Natural childbirth is attested by 1898. Natural life, usually in reference to the duration of life, is from mid-15c.; natural death, one without violence or accident, is from mid-15c. To die of natural causes is from 1570s.

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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. [W.E.H. Lecky, "A History of European Morals," 1869]
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natural-born (adj.)

"so by nature, born so," 1580s, from natural (adj.) + born.

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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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pit (n.1)

Old English pytt (Kentish *pet), "natural or man-made depression in the ground, water hole, well; grave," from Proto-Germanic *putt- "pool, puddle" (source also of Old Frisian pet, Old Saxon putti, Old Norse pyttr, Middle Dutch putte, Dutch put, Old High German pfuzza, German Pfütze "pool, puddle"), an early borrowing from Latin puteus "well, pit, shaft."

The Latin word is perhaps from PIE root *pau- (2) "to cut, strike, stamp," but there are phonetic and sense objections.

Short u makes it impossible to directly derive puteus from paviō 'to strike'. It might be related to putāre 'to prune', but this is semantically less attractive, and the suffix -eus can then hardly be interpreted as indicating a material. Therefore, puteus may well be a loanword. [de Vaan]

Meaning "abode of evil spirits, hell" is attested from late 12c.  Meaning "very small depression or dent in the surface of an object" is from early 15c. The anatomical sense of "natural depression or hollow in some part of the body" is by late 13c,; the pit of the stomach (1650s) is so called from the slight depression there between the ribs; earlier words for it were breast-pit (late 14c.), heart-pit (c. 1300).

The meaning "part of a theater on the floor of the house, lower than the stage," is from 1640s; the sense of "that part of the floor of an exchange where business is carried on" is by 1903, American English. The pit dug under a large engine or other piece of machinery to allow workers to examine or repair it is attested by 1839; this later was extended in auto racing to "area at the side of a track where cars are serviced and repaired" (by 1912).

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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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