Etymology
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libido (n.)
"psychic drive or energy, usually associated with sexual instinct," 1892, carried over untranslated in English edition of Krafft-Ebing's "Psychopathia Sexualis"; and used in 1909 in A.A. Brill's translation of Freud's "Selected Papers on Hysteria" (Freud's use of the term led to its popularity); from Latin libido, lubido "desire, eagerness, longing; inordinate desire, sensual passion, lust," from libere "to be pleasing, to please," from PIE root *leubh- "to care, desire, love" (source also of love).
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fetishism (n.)

1801, "worship of fetishes," from fetish + -ism. Expanded in use by Comte taking it to denote a general type of primitive religion (animism). In the purely psycho-sexual sense, first recorded 1897 in writings of Henry Havelock Ellis (1859-1939).

In certain perversions of the sexual instinct, the person, part of the body, or particular object belonging to the person by whom the impulse is excited, is called the fetish of the patient. [E. Morselli in "Baldwin Dictionary of Philosophy," 1901]

Related: Fetishist (1845; psycho-sexual sense from 1897); fetishistic.

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lonesome (adj.)

"drearily solitary; secluded from society; dejected from want of company," 1640s, from lone (adj.) + -some (1). Related: Lonesomeness. An older adjective was loneful (1560s).

Loneliness expresses the uncomfortable feelings, the longing for society, of one who is alone. Lonesomeness may be a lighter kind of loneliness, especially a feeling less spiritual than physical, growing out of the animal instinct for society and the desire of protection, the consciousness of being alone .... Lonesomeness, more often than loneliness, may express the impression made upon the observer. [Century Dictionary]
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herd (n.1)
Old English heord "herd, flock, company of domestic animals," also, rarely, "a keeping, care, custody," from Proto-Germanic *herdo (source also of Old Norse hjorð, Old High German herta, German Herde, Gothic hairda "herd"), from PIE *kerdh- "a row, group, herd" (source also of Sanskrit śárdhah "herd, troop," Old Church Slavonic čreda "herd," Greek korthys "heap," Lithuanian kerdžius "shepherd"). Of any animals, wild or domestic, from c. 1200; of people, often in a disparaging sense, from c. 1400. Herd instinct in psychology is first recorded 1886.
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pyromania (n.)

form of insanity marked by a mania for destroying things by fire, 1840, from pyro- "fire" + mania "madness, frenzy." Used in German in the 1830s.

The propensity which leads an insane person to accomplish his purpose by burning, has been considered to merit particular notice, and to constitute a variety of monomania. Dr. Marc, of France, has published a memoir on the subject; he gives the name of pyromania to it, and considers that, like other insane propensities, it may be the result of instinct, or it may be the result of delusion—reasoning upon erroneous principles. [Alexander Morrison, M.D., "The Physiognomy of Mental Diseases," London, 1840]

An older word for it was incendiarism.

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pants (n.)

"trousers, drawers," 1840, see pantaloons. The word was limited to vulgar and commercial use at first.

I leave the broadcloth,—coats and all the rest,—
The dangerous waistcoat, called by cockneys "vest,"
The things named "pants" in certain documents,
A word not made for gentlemen, but "gents";
[Oliver Wendell Holmes, "Urania: A Rhymed Lesson," 1846]

Colloquial singular pant is attested from 1893. To wear the pants "be the dominant member of a household" is by 1931. To do something by the seat of (one's) pants "by human instinct" is from 1942, originally of pilots, perhaps with some notion of being able to sense the condition and situation of the plane by engine vibrations, etc. To be caught with (one's) pants down "discovered in an embarrassing condition" is from 1932.

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paraphilia (n.)

"sexual perversion, deviate desires," 1913, from German paraphilie (by 1903), apparently coined by Austrian ethnologist Friedrich Salomo Krauss (1859-1938) as meaning "inverted erotic instinct," from Greek para- "beside, aside" (see para- (1)) + philos "loving" (see -phile). Popularized in psychology circles in English from c. 1918 in translation of work by Viennese-born psychotherapist Wilhelm Stekel (1868-1940); not in widespread use until 1950s. It was added to the "Diagnostic and Statistical Manual of Mental Disorders" in 1980 as a morally neutral and more dignified label than perversion, to which it is nonetheless etymologically similar. Related: Paraphiliac; paraphilic.

The neurotic whose accompanying fancies always lead into forbidden ground (and this is what constitutes the guilt feeling of pollutions) fights against masturbation [pollutions] because it is connected with incest fancies, criminal desires, perversions, or as F.S. Krauss calls them, paraphilias. [Wm. J. Robinson, M.D., "Masturbation — Injurious or Harmless," "American Journal of Urology," May 1913]
Krauss bereichert uns um das neue Wort "Paraphilie" anstelle der "Psychopathie," ein fortschrittlich-oppositionelles Wort zwar, aber auch nur ein Wort und als Aufklärung etwa so bedeutsam wie "Seitensprünge." ["Rezensionen" über die "Anthropophyteia Jahrbücher," Leipzig, 1907]
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octopus (n.)

1758, genus name of a type of eight-armed cephalopod mollusks, from Latinized form of Greek oktōpous, literally "eight-foot," from oktō "eight" (see eight) + pous "foot," from PIE root *ped- "foot."

The more usual Greek word seems to have been polypous (also pōlyps), from polys "many" + pous, but for this word Thompson ["Glossary of Greek Fishes," 1947] suggests folk-etymology and a non-Hellenic origin.

The classically correct Greek plural (had the word been used in this sense in ancient Greek) would be octopodes. Octopi (1817) regards the -us in this word as the Latin noun ending that takes -i in plural. Like many modern scientific names of creatures, it was formed in Modern Latin from Greek elements, so it might be allowed to partake of Latin grammar in forming the plural. But it probably is best to let such words follow the grammar of the language that uses them, and octopuses probably works best in English (unless one wishes also to sanction diplodoci for the dinosaurs).

Used figuratively since at least 1882 of powers having far-reaching influence (usually as considered harmful and destructive). To the ancients, the octopus was crafty and dangerous, thrifty (stores food in its nest), and proverbial of clever and adaptable men, based on the animal's instinct of changing color when frightened or for disguise.

It also was thought to be amphibious and to climb trees near shores to steal grapes and olives (the giant ones were said to raid whole warehouses). Thompson writes that "the eggs look remarkably like ripe olives; hence the story." 

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Valentine (n.)

mid-15c., "sweetheart chosen on St. Valentine's Day," from Late Latin Valentinus, the name of two early Italian saints (from Latin valentia "strength, capacity;" see valence). Choosing a sweetheart on this day originated 14c. as a custom in English and French court circles. Meaning "letter or card sent to a sweetheart" first recorded 1824. The romantic association of the day is said to be from it being around the time when birds choose their mates.

For this was on seynt Volantynys day
Whan euery bryd cometh there to chese his make.
[Chaucer, "Parlement of Foules," c. 1381]

Probably the date was the informal first day of spring in whatever French region invented the custom (many surviving medieval calendars reckon the start of spring on the 7th or 22nd of February). No evidence connects it with the Roman Lupercalia (an 18c. theory) or to any romantic or avian quality in either of the saints. The custom of sending special cards or letters on this date flourished in England c. 1840-1870, declined around the turn of the 20th century, and revived 1920s.

To speak of the particular Customs of the English Britons, I shall begin with Valentine's Day, Feb. 14. when young Men and Maidens get their several Names writ down upon Scrolls of Paper rolled up, and lay 'em asunder, the Men drawing the Maidens Names, and these the Mens; upon which, the Men salute their chosen Valentines and present them with Gloves, &c. This Custom (which sometimes introduces a Match) is grounded upon the Instinct of Animals, which about this Time of the Year, feeling a new Heat by the approach of the Sun, begin to couple. ["The Present State of Great Britain and Ireland" London, 1723]
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