also ox-bow, early 14c., ox-boue, "bow-shaped wooden collar for an ox," from ox + bow (n.1). Meaning "semicircular bend in a river" is from 1797, American English (New England), so called from the resemblance of the shape. The meaning "curved lake left after an oxbow meander has been cut off by a change in the river course" is from 1898.
"arc of prismatic colors formed by the refraction of light rays by drops of rain or vapor," Middle English rein-bowe, from Old English renboga; see rain (n.) + bow (n.). Common Germanic compound (Old Frisian reinboga, Old Norse regnbogi, Swedish regenbåge, Dutch regenboog, German Regenbogen). The American rainbow trout (1876) is so called for its resplendent colors. Old English also had scurboga "shower-bow."
Originally architectural in English; transferred by early 15c. to anything having a curved form (eyebrows, feet, etc.). The commemorative or monumental arch is attested in English from late 14c. Compare Middle English Seinte Marie Chirche of the Arches (c. 1300) in London, later known as St. Mary-le-Bow, site of an ecclesiastical court, so called for the arches that supported its steeple (the modern church is by Sir Christopher Wren, rebuilt after the Great Fire of 1666).
This has Germanic cognates in Gothic arhvazna, Old English earh, Old Norse ör "arrow," from Proto-Germanic *arkw-o- "belonging to a bow." It also has cognates in Greek arkeuthos, Latvian ercis "juniper," Russian rakita, Czech rokyta, Serbo-Croatian rakita "brittle willow." De Vaan sees an Italo-Germanic word for "bow" which can be connected with Balto-Slavic and Greek words for "willow" and "juniper" "under the well-founded assumption that the flexible twigs of juniper or willow were used as bows." The Balto-Slavic and Greek forms point to *arku-; "as with many plant names, this is likely to be a non-IE loanword." Electrical sense is from 1821.
1660s, from French toxique and directly from Late Latin toxicus "poisoned," from Latin toxicum "poison," from Greek toxikon (pharmakon) "(poison) for use on arrows," from toxikon, neuter of toxikos "pertaining to arrows or archery," and thus to a bow, from toxon "bow," probably from a Scythian word that also was borrowed into Latin as taxus "yew." Watkins suggests a possible source in Iranian taxša- "bow," from PIE *tekw- "to run, flee." As a noun from 1890. Toxic waste is by 1888 in medicine, "toxin;" by 1955 as "chemical or radioactive waste."