c. 1300, "injustice;" 1827, "absense of reason," from un- (1) "not" + reason (n.).
Entries linking to unreason
un- 1
prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").
The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.
It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.
reason n.
c. 1200, resoun, "the intellectual faculty that adopts actions to ends," also "statement in an argument, statement of explanation or justification," from Anglo-French resoun, Old French raison "course; matter; subject; language, speech; thought, opinion," from Latin rationem (nominative ratio) "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think" (from PIE root *re- "to reason, count").
Meaning "sanity; degree of intelligence that distinguishes men from brutes" is recorded from late 13c.; that of "that which recommends itself to enlightened intelligence, a reasonable view of a matter" is from c. 1300.
The sense of "grounds for action, motive, cause of an event" is from c. 1300. The Middle English sense of "meaning, signification" (early 14c.) is preserved in the phrase rhyme or reason. Phrase it stands to (or with) reason is from 1520s. A reason of state (1610s) is a purely political grounds for action.
The Enlightenment gave reason its focused sense of "intelligence considered as having universal validity ... so that it is not something that belongs to any person, but is something partaken of, a sort of light in which every mind must perceive" [Century Dictionary]. Reason itself has long been personified, typically as a woman. Age of Reason "the European Enlightenment" is first recorded 1794 as the title of Tom Paine's book.
Reason is never a root, neither of act nor desire.