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).
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.
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]
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.
"restriction to limited allotments," 1865, verbal noun from ration (v.). Specifically of restrictions during wartime from 1917, in reference to conditions in England during the late stages of World War I.
"thin rope," especially, on sailing ships, one of a series of small ropes or lines which form the steps of ladders for going aloft, late 15c., ratling, also ratlin, a word of obscure etymology. Compare Dutch weeflijn, German Webeleine "web line." The spelling ratline is attested from 1773, perhaps by folk etymology influence of rat (n.) + line (n.), "a seamen's jocular name, as if forming ladders for the rats to climb by" [Century Dictionary].