late 14c., racional, "pertaining to or springing from reason;" mid-15c., of persons, "endowed with reason, having the power of reasoning," from Old French racionel and directly from Latin rationalis "of or belonging to reason, reasonable," from ratio (genitive rationis) "reckoning, calculation, reason" (see ratio).
In arithmetic, "expressible in finite terms," 1560s. Meaning "conformable to the precepts of practical reason" is from 1630s. Related: Rationally. It is from the same source as ratio and ration; the sense in rational is aligned with that in related reason (n.), which got deformed in French.
1650s, "exposition of principles," from Late Latin rationale, noun use of neuter of Latin rationalis "of reason" (see rational). Hence, "fundamental reason, the rational basis or motive of anything" (1680s).
1767, "explain in a rational way, make conformable to reason," from rational + -ize. The psychological sense of "to give an explanation that conceals true motives" dates from 1922, on the notion of "cause to appear reasonable or socially acceptable." In 19c. Newman and Browning also used it with an intransitive sense of "think for oneself, employ one's reason as the supreme test." Related: Rationalized; rationalizer; rationalizing.
1620s, "one who follows reason and not authority in thought or speculation," especially "physician whose treatment is based on reasoning," from rational + -ist. In theology/philosophy, "one who applies rational criticism to the claims of supernatural authority or revelation," 1640s. This sense shades into that of "one who believes that human reason, properly employed, renders religion superfluous." Related: Rationalistic; rationalism (1800 in medicine; 1827 in theology, "adherence to the supremacy of reason in matters of belief or conduct;" by 1876 in general use).
1620s, "quality of having reason;" 1650s, "fact of being agreeable to reason," from French rationalité and directly from Late Latin rationalitas "reasonableness, rationality" (also source of Spanish racionalidad, Italian razionalita), from Latin rationalis "of or belonging to reason, reasonable" (see rational). Middle English had racionabilite "the faculty of reason" (early 15c.), from Latin rationabilitas.
late 15c., "not endowed with reason" (of beasts, etc.), from Latin irrationalis/inrationalis "without reason, not rational," from assimilated form of in- "not, opposite of" (see in- (1)) + rationalis "of or belonging to reason, reasonable" (see rational (adj.)).
Meaning "illogical, absurd" is attested from 1640s. Related: Irrationally. The mathematical sense "inexpressible in ordinary numbers" is from late 14c. in English, from use of the Latin word as a translation of Greek alogon in Euclid.
1825, "a rendering rational, act of subjection to rational tests or principles," from rationalize + -ation. The specific sense in psychology in reference to subconscious means to justify behavior to make it seem rational or socially acceptable is by 1908.
Of the three works now on our table, the two which we have placed first have these laudable objects in view; an improvement on the former versions of the Psalms as compositions, and the rationalization, if we may so speak, of our Church psalmody. [The British Critic, London, Jan.-June 1825]
"excuse for being," 1864, first recorded in letter of J.S. Mill, from French raison d'être, literally "rational grounds for existence."
monstrous bird, rational and ancient, in Persian mythology, 1786 ("Vathek"), from Persian simurgh, from Pahlavi sin "eagle" + murgh "bird." Compare Avestan saeno merego "eagle," Sanskrit syenah "eagle," Armenian cin "kite." The thing is probably identical with the roc (q.v.).