1520s, "oblique or diagonal line," from French biais "a slant, a slope, an oblique," also figuratively, "an expedient, means" (13c., originally in Old French a past-participle adjective, "sideways, askance, against the grain"), a word of unknown origin. Probably it came to French from Old Provençal biais, which has cognates in Old Catalan and Sardinian, and is possibly via Vulgar Latin *(e)bigassius from Greek epikarsios "athwart, crosswise, at an angle," from epi "upon" (see epi-) + karsios "oblique" (from PIE *krs-yo-, suffixed form of root *sker- (1) "to cut").
In the old game of bowls, it was a technical term used in reference to balls made with a greater weight on one side (1560s), causing them to curve toward one side; hence the figurative use "a one-sided tendency of the mind" (1570s), and, at first especially in law, "undue propensity or prejudice."
The bias of education, the bias of class-relationships, the bias of nationality, the political bias, the theological bias—these, added to the constitutional sympathies and antipathies, have much more influence in determining beliefs on social questions than has the small amount of evidence collected. [Herbert Spencer, "The Study of Sociology," 1873]
For what a man had rather were true he more readily believes. Therefore he rejects difficult things from impatience of research; sober things, because they narrow hope; the deeper things of nature, from superstition; the light of experience, from arrogance and pride, lest his mind should seem to be occupied with things mean and transitory; things not commonly believed, out of deference to the opinion of the vulgar. Numberless in short are the ways, and sometimes imperceptible, in which the affections colour and infect the understanding. [Francis Bacon, "Novum Organum," 1620]
late 14c., "full of joy, merry; light-hearted, carefree;" also "wanton, lewd, lascivious" (late 12c. as a surname, Philippus de Gay), from Old French gai "joyful, happy; pleasant, agreeably charming; forward, pert; light-colored" (12c.; compare Old Spanish gayo, Portuguese gaio, Italian gajo, probably French loan-words). Ultimate origin disputed; perhaps from Frankish *gahi (related to Old High German wahi "pretty"), though not all etymologists accept this.
Meaning "stately and beautiful; splendid and showily dressed" is from early 14c. Of things, "sumptuous, showy, rich, ornate," mid-14c. of colors, etc., "shining, glittering, gleaming, bright, vivid," late 14c.; of persons, "dressed up, decked out in finery," late 14c.
In the English of Yorkshire and Scotland formerly it could mean "moderately, rather, considerable" (1796; compare sense development in pretty (adj.)).
The word gay by the 1890s had an overall tinge of promiscuity -- a gay house was a brothel. The suggestion of immorality in the word can be traced back at least to the 1630s, if not to Chaucer:
But in oure bed he was so fressh and gay
Whan that he wolde han my bele chose.
Slang meaning "homosexual" (adj.) begins to appear in psychological writing late 1940s, evidently picked up from gay slang and not always easily distinguished from the older sense:
After discharge A.Z. lived for some time at home. He was not happy at the farm and went to a Western city where he associated with a homosexual crowd, being "gay," and wearing female clothes and makeup. He always wished others would make advances to him. [Rorschach Research Exchange and Journal of Projective Techniques, 1947, p.240]
The association with (male) homosexuality likely got a boost from the term gay cat, used as far back as 1893 in American English for "young hobo," one who is new on the road, also one who sometimes does jobs.
"A Gay Cat," said he, "is a loafing laborer, who works maybe a week, gets his wages and vagabonds about hunting for another 'pick and shovel' job. Do you want to know where they got their monica (nickname) 'Gay Cat'? See, Kid, cats sneak about and scratch immediately after chumming with you and then get gay (fresh). That's why we call them 'Gay Cats'." [Leon Ray Livingston ("America's Most Celebrated Tramp"), "Life and Adventures of A-no. 1," 1910]
Quoting a tramp named Frenchy, who might not have known the origin. Gay cats were severely and cruelly abused by "real" tramps and bums, who considered them "an inferior order of beings who begs of and otherwise preys upon the bum -- as it were a jackal following up the king of beasts" [Prof. John J. McCook, "Tramps," in "The Public Treatment of Pauperism," 1893], but some accounts report certain older tramps would dominate a gay cat and employ him as a sort of slave. In "Sociology and Social Research" (1932-33) a paragraph on the "gay cat" phenomenon notes, "Homosexual practices are more common than rare in this group," and gey cat "homosexual boy" is attested in Noel Erskine's 1933 dictionary of "Underworld & Prison Slang" (gey is a Scottish variant of gay).
The "Dictionary of American Slang" reports that gay (adj.) was used by homosexuals, among themselves, in this sense at least since 1920. Rawson ["Wicked Words"] notes a male prostitute using gay in reference to male homosexuals (but also to female prostitutes) in London's notorious Cleveland Street Scandal of 1889. Ayto ["20th Century Words"] calls attention to the ambiguous use of the word in the 1868 song "The Gay Young Clerk in the Dry Goods Store," by U.S. female impersonator Will S. Hays, but the word evidently was not popularly felt in this sense by wider society until the 1950s at the earliest.
"Gay" (or "gai") is now widely used in French, Dutch, Danish, Japanese, Swedish, and Catalan with the same sense as the English. It is coming into use in Germany and among the English-speaking upper classes of many cosmopolitan areas in other countries. [John Boswell, "Christianity, Social Tolerance, and Homosexuality," 1980]
As a teen slang word meaning "bad, inferior, undesirable," without reference to sexuality, from 2000.
1530s, "action, a thing performed, anything done, a deed," good or evil but in 16c.-17c. commonly "evil deed, crime;" from Latin factum "an event, occurrence, deed, achievement," in Medieval Latin also "state, condition, circumstance" (source also of Old French fait, Spanish hecho, Italian fatto), etymologically "a thing done," noun use of neuter of factus, past participle of facere "to do" (from PIE root *dhe- "to set, put").
An earlier adaptation of the Old French word that also became feat. The older senses are mostly obsolete but somewhat preserved in such phrases as after the fact, originally legal, "after the crime." Also compare matter-of-fact.
The modern, empirical, sense of "thing known to be true, a real state of things, what has really occurred or is actually the case," as distinguished from statement or belief, is from 1630s, from the notion of "something that has actually occurred." The particular concept of the scientific, empirical fact ("a truth known by observation or authentic testimony") emerged in English 1660s, via Hooke, Boyle, etc., in The Royal Society, as part of the creation of the modern vocabulary of knowledge (along with theory, hypothesis, etc.); in early 18c. it was associated with the philosophical writings of Hume. Middle English thus lacked the noun and the idea of it; the closest expression being perhaps thing proved (c.1500).
Hence facts "real state of things;" in fact "in reality" (1707). By 1729, fact was being used of "something presented as a fact but which might be or is false."
By fact is also often meant a true statement, a truth, or truth in general ; but this seems to be a mere inexactness of language .... Fact, as being special, is sometimes opposed to truth, as being universal ; and in such cases there is an implication that facts are minute matters ascertained by research, and often inferior in their importance for the formation of general opinions, or for the general description of phenomena, to other matters which are of familiar experience. [Century Dictionary]
Facts of life is by 1854 as "the stark realities of existence;" by 1913 it had also acquired a more specific sense of "knowledge of human sexual functions." The alliterative pairing of facts and figures for "precise information" is by 1727.
Facts and Figures are the most stubborn Evidences; they neither yield to the most persuasive Eloquence, nor bend to the most imperious Authority. [Abel Boyer, "The Political State of Great Britain," 1727]
Facts are stubborn things is in an 1822 translation of the popular novel "Gil Blas" (earlier English translations have facts speak for themselves).