Online Etymology Dictionary

rondo (n.): 1797, "musical composition of one principal theme, which is repeated at least once," from Italian rondo, from French rondeau, rondel, from Old French rondel "little round" (see rondel).
ronin (n.): "masterless man, outcast, outlaw," 1871, from Japanese, from ro "wave" + nin "man."
roo (n.): Australian colloquial shortening of kangaroo, attested from 1904.
rood (n.): Old English rod "pole," varying from 6 to 8 yards; also "cross," especially that upon which Christ suffered; "crucifix," especially a large one; also a measure of land, properly 40 square poles or perches, from Proto-Germanic *rod- (source also of Old Saxon ruoda "stake, pile, cross," Old Frisian rode, Middle Dutch roede, Old High German ruota, German Rute "rod"), from PIE *ret- "post" (source also of Latin ratis "raft," retae "trees standing on the bank of a stream;" Old Church Slavonic ratiste "spear, staff;" Lithuanian rekles "scaffolding"). Probably not connected with rod.
roof (n.): Old English hrof "roof, ceiling, top, summit; heaven, sky," also figuratively, "highest point of something," from Proto-Germanic *khrofam (source also of Old Frisian rhoof "roof," Middle Dutch roof, rouf "cover, roof," Dutch roef "deckhouse, cabin, coffin-lid," Middle High German rof "penthouse," Old Norse hrof "boat shed").

No apparent connections outside Germanic. "English alone has retained the word in a general sense, for which the other languages use forms corresponding to OE. þæc thatch" [OED]. Roof of the mouth is from late Old English. Raise the roof "create an uproar" is attested from 1860, originally in U.S. Southern dialect.
roof (v.): early 15c., from roof (n.). Related: Roofed; roofing.
roof-top (n.): also rooftop, 1610s, from roof (n.) + top (n.1).
roofer (n.): "one who makes or repairs roofs," 1835, agent noun from roof (v.).
rooibos (n.): 1911, from Afrikaans rooibos, literally "red bush," from rooi "red," from Dutch roi (see red (adj.1)) + bos "bush" (see bush (n.1)).
rook (n.1): "European crow," Old English hroc, from Proto-Germanic *khrokaz (source also of Old Norse hrokr, Middle Dutch roec, Dutch roek, Middle Swedish roka, Old High German hruoh "crow"), possibly imitative of its raucous voice (compare Gaelic roc "croak," Sanskrit kruc "to cry out"). Used as a disparaging term for persons since at least c. 1500, and extended by 1570s to mean "a cheat," especially at cards or dice.
rook (n.2): chess piece, c. 1300, from Old French roc, from Arabic rukhkh, from Persian rukh, of unknown meaning, perhaps somehow related to the Indian name for the piece, rut, from Hindi rath "chariot." Confused in Middle English with roc.
rook (v.): "to defraud by cheating" (originally especially in a game), 1590s, from rook (n.1) in some sense (such as "a gull, simpleton," but this is not attested until 17c.). Related: Rooked; rooking.
rookery (n.): "colony of rooks," 1725, from rook (n.1) + -ery.
rookie (n.): "raw recruit," 1892 in that spelling, popularized by Kipling's "Barrack-Room Ballads," of uncertain origin, perhaps from recruit, influenced by rook (n.1) in its secondary sense, suggesting "easy to cheat." Barrère ["A Dictionary of Slang, Jargon & Cant," 1890] has "Rookey (army), a recruit; from the black coat some of them wear," so perhaps directly from rook (n.1). Came into general use in American English during the Spanish-American War.
The rapid growth of a word from a single seed transplanted in a congenial soil is one of the curiosities of literature. Take a single instance. A few weeks ago there was not one American soldier in a thousand who knew there was such a word as "rookey." To-day there are few soldiers and ex-soldiers who have not substituted it for "raw recruit." ["The Midland Monthly," December 1898]
room (n.): Old English rum "space" (extent or time); "scope, opportunity," from Proto-Germanic *ruman (source also of Old Norse, Old Saxon, Old High German, Gothic rum, German Raum "space," Dutch ruim "hold of a ship, nave"), nouns formed from Germanic adjective *ruma- "roomy, spacious," from PIE root *reue- (1) "to open; space" (source also of Avestan ravah- "space," Latin rus "open country," Old Irish roi, roe "plain field," Old Church Slavonic ravinu "level," Russian ravnina "a plain"). Old English also had a frequent adjective rum "roomy, wide, long, spacious."

Original sense preserved in make room "clear space for oneself" (late 14c.); meaning "chamber, cabin" first recorded early 14c. as a nautical term, and first applied mid-15c. to chambers within houses. The Old English word for this was cofa, ancestor of cove. Room-service is attested from 1913; room-temperature from 1879. Roomth "sufficient space" (1530s) now is obsolete.
room (v.): "to occupy rooms" (especially with another) as a lodger," 1828, from room (n.). Related: Roomed; rooming. Rooming-house is from 1889. In Old English (rumian) and Middle English the verb meant "become clear of obstacles; make clear of, evict."
roomer (n.): "one who hires a room," 1871, agent noun from room (v.).
roommate (n.): also room-mate, 1789, American English, from room (n.) + mate (n.). Short form roomie is from 1918.
roomy (adj.): "roomsome," 1620s, from room (n.) + -y (2). Related: Roominess.
roose (v.): c. 1200, "to boast;" c. 1300, "to praise," Scottish dialect, from Old Norse hrosa "to boast of, to praise." Related: Roosed; roosing. Also as a noun from c. 1200.
Roosevelt: the family in America originally bore the name Van Roosevelt, "of the field of roses," descriptive of their estates in Holland. Claes Martenszen Van Rosenvelt, born August 1649, emigrated to New Amsterdam. His son (1653) and all his descendants dropped the "Van." Related: Rooseveltian.
roost (n.): late Old English hrost "wooden framework of a roof, perch for domestic fowl," from Proto-Germanic *hro(d)-st- (source also of Old Saxon hrost "framework of a roof, attic," Middle Dutch, Flemish, Dutch roest "roost," Old Norse hrot, Gothic hrot "roof," of unknown origin. Exact relationship and ulterior connections unknown. Extended sense "hen-house" is from 1580s. To rule the roost is recorded from 1769.
roost (v.): 1520s, from roost (n.). Related: Roosted; roosting. Chickens come home to roost in reference to eventual consequences of bad actions attested from 1824; the original proverb seems to have been curses, like chickens, come home to roost.
rooster (n.): 1772, agent noun from roost (v.); earlier roost cock, c. 1600, in sense of "the roosting bird." Favored in the U.S. originally as a puritan alternative to cock (n.) after it had acquired the secondary sense "penis" (and compare roach).
root (n.): "underground part of a plant," late Old English rot, from a Scandinavian source akin to Old Norse rot "root," figuratively "cause, origin," from Proto-Germanic *wrot (source also of Old English wyrt "root, herb, plant," Old High German wurz, German Wurz "a plant," Gothic waurts "a root," with characteristic Scandinavian loss of -w- before -r-), from PIE *wrad- (see radish (n.), and compare wort). The usual Old English words for "root" were wyrttruma and wyrtwala.

Figurative use is from c. 1200. Of teeth, hair, etc., from early 13c. Mathematical sense is from 1550s. Philological sense from 1520s. Slang meaning "penis" is recorded from 1846. In African-American vernacular use, "a spell effected by magical properties of roots," 1935. To take root is from 1530s. Root beer, made from the extracts of various roots, first recorded 1841, American English; root doctor is from 1821. Root cap is from 1875.
root (v.1): "dig with the snout," 1530s, from Middle English wroten "dig with the snout," from Old English wrotan "to root up," from Proto-Germanic *wrot- (source also of Old Norse rota, Swedish rota "to dig out, root," Middle Low German wroten, Middle Dutch wroeten, Old High German ruozian "to plow up"), from PIE root *wrod- "to root, gnaw."

Associated with the verb sense of root (n.). Extended sense of "poke about, pry" first recorded 1831. Phrase root hog or die "work or fail" first attested 1834, American English (in works of Davey Crockett, who noted it as an "old saying"). Reduplicated form rootin' tootin' "noisy, rambunctious" is recorded from 1875.
root (v.2): "cheer, support," 1889, American English, originally in a baseball context, probably from root (v.1) via intermediate sense of "study, work hard" (1856). Related: Rooted; rooting.
root (v.3): "fix or firmly attach by roots" (often figurative), early 13c., from root (n.); sense of "pull up by the root" (now usually uproot) also is from late 14c. Related: Rooted; rooting.
rootless (adj.): late 14c., from root (n.) + -less. Related: Rootlessly; rootlessness.
rope (n.): Old English rap "rope, cord, cable," from Proto-Germanic *raipaz (source also of Old Norse reip, West Frisian reap, Middle Dutch, Dutch reep "rope," Old Frisian silrap "shoe-thong," Gothic skauda-raip "shoe-lace," Old High German, German reif "ring, hoop"). Technically, only cordage above one inch in circumference and below 10 (bigger-around than that is a cable). Nautical use varies. Finnish raippa "hoop, rope, twig" is a Germanic loan-word.

To know the ropes (1840, Dana) originally is a seaman's term. Phrase on the ropes "defeated" is attested from 1924, a figurative extension from the fight ring, where ropes figure from 1829. To be at the end of (one's) rope "out of resources and options" is first attested 1680s. Formerly also in many slang and extended uses related to punishment by hanging, such as John Roper's window "a noose," rope-ripe "deserving to be hanged," both 16c. To give someone (enough) rope (to hang himself) is from 1650s.
rope (v.): c. 1300, "bind with a rope," from rope (n.). Meaning "mark off with rope" is from 1738; to rope (someone or something) in is from 1848. Related: Roped; roping.
ropy (adj.): late 15c., from rope (n.) + -y (2). Related: Ropiness.
roque: 1899, "arbitrary alteration of croquet" [OED].
Roquefort: type of cheese, 1837, from the village in the southwest of France, where it originally was made. Reference to salad dressing made from this kind of cheese is from 1943.
Rorschach: 1927, in reference to a personality test using ink blots, developed by Swiss psychiatrist Hermann Rorschach (1885-1922). The town so named on the Swiss side of Lake Constance is from an early form of German Röhr "reeds" + Schachen "lakeside."
Rosa: fem. proper name, from Latin Rosa, literally "rose" (see rose (n.1)).
rosacea (n.): 1876, short for acne rosacea (1833), from fem. of Latin rosaceus "rose-colored" (see rose (n.1)).
rosary (n.): "rose garden," mid-15c., from Latin rosarium "rose garden," in Medieval Latin also "garland; string of beads; series of prayers," from noun use of neuter of rosarius "of roses," from rosa "rose" (see rose (n.1)).

The sense of "series of prayers" is 1540s, from Middle French rosaire, a figurative use of the word meaning "rose garden," on the notion of a "garden" of prayers. This probably embodies the medieval conceit of comparing collections to bouquets (compare anthology and Medieval Latin hortulus animae "prayerbook," literally "little garden of the soul"). Sense transferred 1590s to the strings of beads used as a memory aid in reciting the rosary.
Roscius (n.): name of a celebrated Roman actor.
roscoe (n.): "revolver," 1914, criminals' slang, from the proper name, for some reason.
rose (n.1): Old English rose, from Latin rosa (source of Italian and Spanish rosa, French rose; also source of Dutch roos, German Rose, Swedish ros, Serbo-Croatian ruža, Polish róża, Russian roza, Lithuanian rože, Hungarian rózsa, Irish ros, Welsh rhosyn, etc.), probably via Italian and Greek dialects from Greek rhodon "rose" (Aeolic wrodon), probably ultimately related to Iranian root *vrda-.

But Tucker writes: "The rose was a special growth of Macedonia & the Thracian region as well as of Persia, & the Lat. & Gk. names prob. came from a Thraco-Phrygian source." Aramaic warda is from Old Persian; the modern Persian cognate, via the usual sound changes, is gul, source of Turkish gül "rose." Klein proposes a PIE *wrdho- "thorn, bramble."

The form of the English word was influenced by the French. Used as a color name since 1520s. In English civil wars of 15c., the white rose was the badge of the House of York, the red of its rival Lancaster. In the figurative sense, bed of roses is from 1590s. (In 15c. to be (or dwell) in flowers meant "be prosperous, flourish.") To come up roses is attested from 1969; the image, though not the wording, from 1855. To come out smelling like a rose is from 1968. Rose of Sharon (Song of Sol. ii:1) is attested from 1610s and named for the fertile strip of coastal Palestine. The flower has not been identified; used in U.S. since 1847 of the Syrian hibiscus.
rose (n.2): light red wine, 1897, from French vin rosé, literally "pink wine."
rose-colored (adj.): "optimistic," 1854, from rose (n.1) on the notion of "something uncommonly beautiful."
rose-red (adj.): c. 1300, from rose (n.1) + red (adj.1).
rose-water (n.): late 14c., water tinctured with oil of roses, from rose (n.1) + water (n.1). Symbolic of affected delicacy or sentimentalism. Similar formation in Middle Dutch rosenwater, Dutch rozenwater, German Rosenwasser.
roseate (adj.): 1580s, from Latin roseus (see rose (n.1)) + -ate.
rosebud (n.): 1610s, from rose (n.1) + bud (n.). Hence, "young girl in her first bloom, a debutante."
rosemary (n.): late 14c., earlier rosmarine (c. 1300), from Latin rosmarinus, literally "dew of the sea" (compare French romarin), from ros "dew" + marinus (see marine (adj.)). Perhaps so called because it grew near coasts. Form altered in English by influence of rose and Mary.

Latin ros is from PIE *ers- "to be wet" (source also of Lithuanian rasa, Old Church Slavonic rosa "dew," Sanskrit rasah "sap, juice, fluid, essence," Hittite arszi "flows," and perhaps also Rha, Scythian name of the River Volga (see rhubarb)).
Rosetta Stone (n.): discovered 1798 at Rosetta, Egypt; now in British Museum. Dating to 2c. B.C.E., its trilingual inscription helped Jean-François Champollion decipher Egyptian demotic and hieroglyphics in 1822, which opened the way to study of all early Egyptian records. Hence, figurative use of the term to mean "something which provides the key to previously unattainable understanding" (1902). The place name is the European form of Rashid, a name given because it was founded c.800 C.E. by Caliph Harun ar-Rashid.
rosette (n.): "a rose-shaped ornament," especially a bunch or knot of ribbons worn as a decoration, 1790, from French rosette, diminutive of rose "rose" (see rose (n.1)).