1680s, "of or pertaining to the mind as a subject of study;" see psychology + -ical. In early 20c. the sense gradually shifted toward "affecting or pertaining to a person's mental or emotional state." Related: Psychologically. Psychological warfare "use of propaganda, etc., to undermine an enemy's morale or resolve" is recorded from 1940. Psychological moment was in vogue from 1871, from French moment psychologique "moment of immediate expectation of something about to happen."
The original German phrase, misinterpreted by the French & imported together with its false sense into English, meant the psychic factor, the mental effect, the influence exerted by a state of mind, & not a point of time at all, das Moment in German corresponding to our momentum, not our moment. [Fowler]
c. 1200, "medical treatment, cure, healing," also (early 14c.) "substance used in treatment of a disease, medicinal potion or plaster," also used figuratively of spiritual remedies, from Old French medecine (Modern French médicine) "medicine, art of healing, cure, treatment, potion" and directly from Latin medicina "the healing art, medicine; a remedy," also used figuratively.
This is perhaps originally ars medicina "the medical art," from fem. of medicinus (adj.) "of a doctor," from medicus "a physician" (from PIE root *med- "take appropriate measures"); though OED says evidence for this path is wanting and suggests derivation directly from medicus. The sense of "practice, theory, or study of curing, alleviating, or preventing disease in humans" is from mid-14c.
The figurative phrase take (one's) medicine "submit to something disagreeable" is recorded by 1865; that of dose of (one's) own medicine is by 1894. Medicine show "traveling show meant to attract a crowd so patent medicine can be sold to them" is American English, 1938. Medicine ball "stuffed leather ball used for exercise" is from 1889.
It is called a "medicine ball" and it got that title from Prof. [Robert J.] Roberts, now of Springfield, whose fame is widespread, and whose bright and peculiar dictionary of terms for his prescription department in physical culture is taught in every first-class conducted Y.M.C.A. gymnasium in America. Prof. Roberts calls it a "medicine ball" because playful exercise with it invigorates the body, promotes digestion, and restores and preserves one's health. [Scientific American Supplement, March 16, 1889]
"Native North American shaman," by 1801, from adoption of the word medicine in native speech with a sense of "magical influence; something supposed to possess curative, supernatural, or mysterious power." The U.S.-Canadian boundary they called the Medicine Line (attested by 1880), because it conferred a kind of magic protection: punishment for crimes committed on one side of it could be avoided by crossing over to the other. Compare Middle English use of medicine in secondary senses of "moral, psychological, or social remedy; safeguard, defense."
Unless some understanding is arrived at between the American and Canadian Governments that offenders may be promptly and vigorously dealt with, I very much fear that killing and stealing will increase to such an extent that the country along the border will be scarcely habitable. When the Indians are made to understand that the mere fact of "hopping" across the line does not exempt them from punishment, there will be a much greater guarantee of their good behaviour. Now they call the boundary the "Medicine line," because no matter what they have done upon one side they feel perfectly secure after having arrived upon the other. [Report of Superintendent L.N.F. Crozier, Dec. 1880, in "North-West Mounted Police Force Commissioner's Report," 1880]
Hence also medicine bag "pouch containing some article supposed to possess curative or magical powers, worn on the person by native North American people" (1802).
1781, "directed inward" (of the mind, etc.), past-participle adjective from introvert (v.). Psychological sense is from 1915. Other adjectives in the non-psychological sense were introversive (1820), introvertive (1846), introverse (1874).
"having healing or curative properties, suitable for medical use," mid-14c., from Old French medicinal and directly from Latin medicinalis "pertaining to medicine," from medicina "the healing art, medicine; a remedy" (see medicine). Related: Medicinally.