Etymology
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instinct (n.)

early 15c., "a prompting" (a sense now obsolete), from Old French instinct (14c.) or directly from Latin instinctus "instigation, impulse, inspiration," noun use of past participle of instinguere "to incite, impel," from in- "into, in, on, upon" (from PIE root *en "in") + stinguere "prick, goad," from PIE *steig- "to prick, stick, pierce" (see stick (v.)).

Meaning "animal faculty of intuitive perception" is from mid-15c., from notion of "natural prompting." General sense of "natural tendency" is first recorded 1560s.

Instinct is said to be blind--that is, either the end is not consciously recognized by the animal, or the connection of the means with the end is not understood. Instinct is also, in general, somewhat deficient in instant adaptability to extraordinary circumstances. [Century Dictionary]
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instinctual (adj.)
1841, from instinct (Latin instinctus) + -al (1). Related: Instinctually.
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instinctive (adj.)
1640s, from Latin instinct-, past participle stem of instinguere "to incite, impel" (see instinct) + -ive. Related: Instinctively (1610s); instinctiveness. Coleridge uses instinctivity.
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thanatos (n.)
"death instinct," 1935, in Freudian psychology, from Greek thanatos "death" (see thanato-).
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pervert (n.)

1660s, "one who has forsaken a doctrine or system regarded as true, an apostate," from pervert (v.). Psychological sense of "one who has a perversion of the sexual instinct" is attested from 1897 (Havelock Ellis), originally especially of homosexuals.

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fraidy-cat (n.)

"coward, timid person," by 1871, American English slang, from 'fraid (by 1816), childish or dialectal (African, West Indies) pronunciation of afraid, + cat (n.), perhaps in reference to the animals' instinct to scatter when startled. (Scaredy-cat is from 1906).

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pansexual (adj.)

1926 (pansexualism is from 1917), from pan- + sexual. Originally in reference to the view that the sex instinct plays the primary part in all human activity, mental and physical; Freud's critics held this to be his view, and the word became a term of reproach leveled at early psychology. Meaning "not limited in sexual choice" is attested by 1972. Related: Pansexuality.

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killer (n.)
late 15c., agent noun from kill (v.). But a surname, Ric[hard] Le Kyller is attested from 1288. Figurative use from 1550s. Meaning "impressive person or thing" is by 1900 (as an adjective, 1979); reduplicated form killer-diller attested by 1938. Killer whale is from 1854 (earlier simply killer 1725); killer instinct is attested from 1931, originally in boxing.
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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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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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