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. Transferred sense of "predisposition, prejudice" is from 1570s in English.
[A] technical term in the game of bowls, whence come all the later uses of the word. [OED]