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]
late 15c., "to make conditions, stipulate," from condition (n.). Meaning "subject to something as a condition" is from 1520s; sense of "form a prerequisite of" is from 1868. Meaning "to bring to a desired condition" is from 1844; psychological sense of "teach or accustom (a person or animal) to certain habits or responses" is from 1909. Related: Conditioned; conditioning.
mid-14c., condicioun, "particular mode of being of a person or thing," also "a requisite or prerequisite, a stipulation," from Old French condicion "stipulation; state; behavior; social status" (12c., Modern French condition), from Medieval Latin conditionem (nominative conditio), properly condicio "agreement; stipulation; the external position, situation, rank, place, circumstances" of persons, "situation, condition, nature, manner" of things, from condicere "to speak with, talk together, agree upon," in Late Latin "consent, assent," from assimilated form of com "together" (see con-) + dicere "to speak" (from PIE root *deik- "to show," also "pronounce solemnly").
Classical Latin condicio was confused in Late Latin with conditio "a making," from conditus, past participle of condere "to put together." The sense evolution in Latin apparently was from "stipulation" to "situation, mode of being."
Meaning "rank or state with respect to ordered society" is from late 14c. in English. From the notion of "prerequisite" comes the sense of "a restricting or limiting circumstance" (late 14c.). Also in Middle English "personal character, disposition" (mid-14c.).
"morbid fear of everything," attested by 1848 in medical journals for a psychological condition described as "monomania, with fear and terror," from Greek pantos, neuter genitive of pas "all" (see pan-) + -phobia. Earlier (by 1819) it was noted as an old word for "hydrophobia, rabies." Related: Pantophobe; pantophobic.
1923, as a psychological theory (in the nature vs. nurture debate), from environmental + -ism. The ecological sense is from 1972. Related: Environmentalist (n.), 1916 in the psychological sense, 1970 in the ecological sense.
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).
1927, shortening of psychological; 1936 (Raymond Chandler) as shortening of psychopathic (adj.).
"morbid dread of having caught rabies," a psychological condition which sometimes mimicked the actual disease, 1874, Modern Latin, from -phobia + Greek lyssa (Attic lytta) "rabies, canine madness," also the name given to the "worm" of cartilage under a dog's tongue," an abstract word probably literally "wolf-ness" and related to lykos "wolf" (see wolf (n.)); but some see a connection with "light" words, in reference to the glittering eyes of the mad.