Etymology
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rainbow (n.)

"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."

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cactus (n.)

c. 1600, in the classical sense, "cardoon, artichoke," from Latin cactus, from Greek kaktos, name of a type of prickly plant of Sicily (the Spanish artichoke), a "foreign word of unknown origin" [Beekes]. In reference to the green, leafless, spiked American plants from 1769. Linnaeus gave the name to them thinking they were related to the classical plant. Related: Cactal.

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iridescent (adj.)

1784, literally "rainbow-colored," coined from Latin iris (genitive iridis) "rainbow" (see iris). The verb iridesce (1868) is a back-formation. Related: Iridescently.

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peyote (n.)

"mescal cactus," 1849, in reference to the mescal made from it, from Mexican Spanish peyote, from Nahuatl (Aztecan) peyotl, said to mean "caterpillar;" the cactus so called from the downy button on top.

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saguaro (n.)

giant branching columnar species of cactus of the deserts of Arizona and Mexico, 1856, from Mexican Spanish, from a native name of unknown origin, perhaps from Yaqui (Sonoran).

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Tenochtitlan 

former Aztec city, Nahuatl (Aztecan), literally "place of the nopal rock," from tetl "rock" + nuchtli "nopal," a species of cactus sacred to the sun god.

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nopal (n.)

type of Mexican cactus (which supports the cochineal bug), 1730, from American Spanish, from Nahuatl (Aztecan) nopalli.

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iris (n.)

late 14c. as the name of a flowering plant (Iris germanica); early 15c. in reference to the eye membrane, from Latin iris (plural irides) "iris of the eye; iris plant; rainbow," from Greek iris (genitive iridos) "a rainbow;" also "iris plant" and "iris of the eye," a word of uncertain origin, traditionally derived from PIE root *wei- "to bend, turn, twist."

Iris was the name of the minister and messenger of the Olympian gods (especially of Hera), visibly represented by the rainbow (which was regarded as the descent of a celestial messenger). From the oldest parts of the Iliad the word is used of both the messenger and the rainbow.

The eye region was so called (early 15c. in English) for being the part that gives color to the eye; the Greek word was used of any brightly colored circle, "as that round the eyes of a peacock's tail" [Liddell & Scott]. Another sense in Middle English was "prismatic rock crystal." Related: Iridian; iridine.

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Cereus (n.)

cactus genus, 1730, from Latin cereus "waxen, waxy," from cera "wax" (see cero-). So called from its shape, which suggests a wax candle.

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prickly (adj.)

1570s, "spiny, full of sharp points, armed with prickles" (originally of holly leaves), from prickle (n.) + -y (2). Figurative sense of "irritable, quick to anger" is recorded by 1862. Prickly heat "inflammatory disorder of the sweat glands" is from 1736, so called for the sensation; prickly pear, of the fruit of a certain cactus, is from 1760 (earlier prickle pear, 1610s). Related: Prickliness.

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