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]
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.
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.
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]