1854, "a general accord or agreement of different parts in effecting a given purpose," originally a term in physiology; 1861, of persons "a general agreement in opinion;" from Latin consensus "agreement, accord," past participle of consentire "feel together," from assimilated form of com "with, together" (see con-) + sentire "to feel" (see sense (n.)). There is an isolated instance of the word from 1633.
Until modern times used almost exclusively with reference to legal contracts and to the eyes working together reflexively; its sense was extended in the language of sociology and psychology from 1950s (of social groups, non-arranged marriages, etc.) and in the legal discussions of rape and other sex crimes by 1977.
Old English we, first person plural pronoun, "I and another or others," from Proto-Germanic *wejes (source also of Old Saxon wi, Old Norse ver, Danish vi, Old Frisian wi, Dutch wij, Old High German and German wir, Gothic weis "we"), from PIE *we- (source also of Sanskrit vayam, Old Persian vayam, Hittite wesh "we," Old Church Slavonic ve "we two," Lithuanian vedu "we two").
The "royal we" (use of plural pronoun to denote oneself) is at least as old as "Beowulf" (c.725); use by writers to establish an impersonal style is also from Old English; it was especially common 19c. in unsigned editorials, to suggest staff consensus, and was lampooned as such at least since 1853 (see wegotism).
c. 1300, resonable, "having sound judgment, endowed with the faculty of reason," from Old French raisonable, from Latin rationabilis, from ratio "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think" (from PIE root *re- "to reason, count").
Also originally "rational, sane," senses now obsolete. The sense shifted somewhat in Middle English via "due to or resulting from good judgment," then "not exceeding the bounds of common sense."
The meaning "moderate in price" is recorded from 1660s; earlier it meant "moderate in amount" (14c.). Related: Reasonably, which is from late 14c. as "according to reason," c. 1500 as "fairly tolerably;" reasonableness.
The adjective reasonable ... denotes a character in which reason, (taking that word in its largest acceptation,) possesses a decided ascendant over the temper and passions: and implies no particular propensity to a display of the discursive power, if indeed it does not exclude the idea of such a propensity. [Dugald Stewart, "Elements of the Philosophy of the Human Mind," 1856]
What the majority of people consider to be 'reasonable' is that about which there is agreement, if not among all, at least among a substantial number of people; 'reasonable' for most people, has nothing to do with reason, but with consensus. [Erich Fromm, "The Heart of Man," 1968]
In law, "befitting a person of reason or sound sense;" reasonable doubt (1670s) is doubt for which a pertinent reason can be assigned and which prevents conviction in the minds of jurors of the truth of the charge.