Borrowed from French ratiocination, from Latin ratiōcinātiō (“argumentation, reasoning, ratiocination; a syllogism”), from ratiōcinātus (“reckoned”) + -tiō (“suffix forming a noun relating to some action or the result of an action”).
Ratiōcinātus is the perfect passive participle of ratiōcinor (“to compute, reckon; to argue, infer”), from ratiō (“reason, explanation”) (from reor (“to calculate, reckon”), possibly from Proto-Italic *rēōr, from Proto-Indo-European *h₂reh₁- (“to put in order”)) + -cinor, modelled after vāticinor (“to foretell, prophesy”), equivalent to ratiocinate + -ion.
