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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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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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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iridium (n.)
silver-white metallic element, 1804, coined in Modern Latin by its discoverer, English chemist Smithson Tennant (1761-1815) from Greek iris (genitive iridos) "rainbow" (see iris) + chemical ending -ium. So called "from the striking variety of colours which it gives while dissolving in marine acid" [Tennant]
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guppy (n.)
1918, so called about the time they became popular as aquarium fish, from the scientific name (Girardinus guppii), which honored R.J.L. Guppy, the British-born Trinidad clergyman who supplied the first specimen (1866) to the British Museum. The family name is from a place in Dorset. Other early popular names for it were rainbow fish and million fish. The class of streamlined U.S. submarines (1948) is an acronym from greater underwater propulsion power + -y.
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*wei- 

also weiə-, Proto-Indo-European root meaning "to turn, twist, bend," with derivatives referring to suppleness or binding. 

It forms all or part of: ferrule; garland; iridescence; iridescent; iris; iridium; vise; viticulture; wire; withe; withy.

It is the hypothetical source of/evidence for its existence is provided by: Avestan vaeiti- "osier;" Greek itea "willow," iris "rainbow;" Latin viere "to bend, twist," vitis "vine;" Lithuanian vytis "willow twig;" Old Irish fiar, Welsh gwyr "bent, crooked;" Polish witwa, Welsh gwden "willow," Russian vitvina "branch, bough;" Old English wir "metal drawn out into a fine thread." 

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bow (n.1)

"strung, elastic weapon for shooting arrows," Old English boga "archery bow; anything bent or arched, an arch, a rainbow," from Proto-Germanic *bugon (source also of Old Norse bogi, Old Frisian boga, Dutch boog, German Bogen "bow"), from PIE root *bheug- "to bend," with derivatives referring to bent, pliable, or curved objects. The sense of "a looped knot," especially an ornamental one, is from 1540s. The musician's bow (1570s) formerly was curved like the archer's.

The former popularity of the longbow as a characteristic English weapon is attested in expressions such as bow-legged; to have the bent of (one's) bow "know one's intentions or inclinations" (1560s), to shoot in (another's) bow "practice an art other than one's own;" bow-hand "the left hand," hence "on the wrong side, inaccurately;" have two strings to (one's) bow "have more than one means to accomplish something;" draw the long bow "exaggerate, lie."

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father (n.)
Origin and meaning of father

Old English fæder "he who begets a child, nearest male ancestor;" also "any lineal male ancestor; the Supreme Being," and by late Old English, "one who exercises parental care over another," from Proto-Germanic *fader (source also of Old Saxon fadar, Old Frisian feder, Dutch vader, Old Norse faðir, Old High German fatar, German vater; in Gothic usually expressed by atta), from PIE *pəter- "father" (source also of Sanskrit pitar-, Greek pater, Latin pater, Old Persian pita, Old Irish athir "father"), presumably from baby-speak sound "pa." The ending formerly was regarded as an agent-noun affix.

My heart leaps up when I behold
  A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
  Or let me die!
The Child is father of the Man;
I could wish my days to be
Bound each to each by natural piety.
[Wordsworth, 1802]

The classic example of Grimm's Law, where PIE "p-" becomes Germanic "f-." Spelling with -th- (15c.) reflects widespread phonetic shift in Middle English that turned -der to -ther in many words, perhaps reinforced in this case by Old Norse forms; spelling caught up to pronunciation in 1500s (compare mother (n.), weather (n.), hither, gather). As a title of various Church dignitaries from c. 1300; meaning "creator, inventor, author" is from mid-14c.; that of "anything that gives rise to something else" is from late 14c. As a respectful title for an older man, recorded from 1550s. Father-figure is from 1954. Fathers "leading men, elders" is from 1580s.

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red (adj.1)

"of a bright, warm color resembling that of blood or of the highest part of the primery rainbow" [Century Dictionary], Middle English rēd, redde, read, reid, from Old English rēad, used of various shades of purple, crimson, scarlet, pink, etc.; also red clothes, dye, ink, wine, or paint, also "having a ruddy or reddish complexion; red-haired, red-bearded;" from Proto-Germanic *rauthan (source also of Old Norse rauðr, Danish rød, Old Saxon rod, Old Frisian rad, Middle Dutch root, Dutch rood, German rot, Gothic rauþs).

This is reconstructed to be from a PIE root *reudh- "red, ruddy," the only color for which a definite common PIE root word has been found. It also is the root of native ruddy, rust, and, via Latin, ruby, rubric, russet, etc.

Along with dead, bread (n.), lead (n.1), its long vowel shortened in or after Middle English. The surname Read, Reid, Reade, etc. represents the old form of the adjective and retains the original Old English long vowel pronunciation. It corresponds to Brown, Black, White; Red itself being rare as a surname. As the color designation of Native Americans in English from 1580s.

In fixed comparisons, red as blood (Old English), roses (mid-13c.), cherry (c. 1400). From Old English as the color characteristic of inflammation, blistering, etc. Of the complexion, lips, etc., "ruddy, rosy, red" (c. 1200); also of person with a healthy complexion or skin color; to be red in the face as a result of powerful emotion or agitation is by c. 1200; to see red "get angry" is an American English expression attested by 1898.

Red as the characteristic color of "British possessions" on a map is attested from 1885. Red-white-and-blue in reference to American patriotism, from the colors of the flag, is from 1840; in a British context, in reference to the Union flag, 1852.

Red rover, the children's game, attested from 1891. Red ball signifying "express" in railroad jargon is by 1904, originally (1899) a system of moving and tracking freight cars. Red dog, type of U.S. football pass rush, is recorded from 1959 (earlier "lowest grade of flour produced in a mill," by 1889). Red meat, that which is ordinarily served or preferred undercooked, is from 1808; the food of wild beasts, hence its figurative use for something that satisfies a basic appetite (by 1792; popular from late 20c.).

Red shift in spectography is first recorded 1923. Red carpet "sumptuous welcome" is from 1934, but the custom for dignitaries is described as far back as Aeschylus ("Agamemnon"); it also was the name of a type of English moth. Red ant is from 1660s.

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