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1520s, from French biais "slant, slope, oblique," also figuratively, "expedient, means" (13c., originally in Old French a past participle adjective, "sideways, askance, against the grain"), of unknown origin, probably from Old Provençal biais, with cognates in Old Catalan and Sardinian; possibly from Vulgar Latin *(e)bigassius, from Greek epikarsios "athwart, crosswise, at an angle," from epi- "upon" + karsios "oblique," from PIE *krs-yo-, from root *(s)ker- "to cut." It became a noun in Old French. "[A] technical term in the game of bowls, whence come all the later uses of the word" [OED]. Transferred sense of "predisposition, prejudice" is from 1570s in English.
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]
1620s, literal and figurative, from bias (n.). Related: Biased; biasing.