Rational
late Middle English (in the sense ‘having the ability to reason’): from Latin rationalis, from ratio(n- ) ‘reckoning, reason’ (see ratio).
wiktionary
From Old French rationel, rational, from Latin rationalis(“of or belonging to reason, rational, reasonable; having a ratio”), from ratio(“reason; calculation”).
From Old French rational, from Medieval Latin rationale(“a pontifical stole, a pallium, an ornament worn over the chasuble”), neuter of Latin rationalis(“rational”), for which see the first etymology. Translation of λογεῖον(logeîon) or perhaps λόγιον(lógion, “oracle”) in the Septuagint version of Exodus 28.
etymonline
rational (adj.)
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.
