Etymology
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rodent (n.)

"a rodent mammal" 1835 (as an adjective 1833), from Modern Latin Rodentia, the order name, from Latin rodentem (nominative rodens), "the gnawers," present participle of rodere "to gnaw, eat away," which is of uncertain etymology, possibly is from an extended form of PIE root *red- "to scrape, scratch, gnaw." Uncertain connection to Old English rætt (see rat (n.)). They are characterized by having no canine teeth and strong incisors.

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corrode (v.)

late 14c., "to eat away, diminish or disintegrate (something) by gradually separating small bits of it," from Old French corroder (14c.) and directly from Latin corrodere "to gnaw to bits, wear away," from assimilated form of com-, here perhaps an intensive prefix (see com-), + rodere "to gnaw" (see rodent). Figurative use from 1630s. Related: Corroded; corroding.

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erosion (n.)

1540s, from French erosion (16c.), from Latin erosionem (nominative erosio) "a gnawing away," noun of action from past-participle stem of erodere "to gnaw away, consume," from assimilated form of ex "away" (see ex-) + rodere "to gnaw" (see rodent). Related: Erosional.

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raster (n.)

"scanning field," 1934 in electrical engineering, from German Raster "screen, frame," from Latin rastrum "rake," from rasum, from rodere "to scrape" (see rodent). Related: Rasterization; rasterize. From Latin form rastellum comes French râteau "rake," formerly ratel, originally rastel.

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corrosion (n.)

c. 1400, corrosioun, from Old French corrosion and directly from Latin corrosionem (nominative corrosio), noun of action from past-participle stem of corrodere "to gnaw to bits, wear away," from assimilated form of com-, here perhaps an intensive prefix (see com-), + rodere "to gnaw" (see rodent).

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erode (v.)

1610s, "gnaw or eat away" (transitive), a back-formation from erosion, or else from French éroder, from Latin erodere "to gnaw away, consume," from assimilated form of ex "away" (see ex-) + rodere "to gnaw" (see rodent). Intransitive sense "become worn away" is by 1905. Related: Eroded; eroding. Originally of acids, ulcers, etc.; geological sense is from 1830.

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*red- 

*rēd-, Proto-Indo-European root meaning "to scrape, scratch, gnaw."

It forms (possibly) all or part of: abrade; abrasion; corrode; corrosion; erase; erode; erosion; radula; rascal; rase; rash (n.) "eruption of small red spots on skin;" raster; rat; raze; razor; rodent; rostrum; tabula rasa.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit radati "scrapes, gnaws," radanah "tooth;" Latin rodere "to gnaw, eat away," radere "to scrape;" Welsh rhathu "scrape, polish."

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rostrum (n.)

"pulpit or platform from which a speaker addresses an audience," 1540s, originally in an ancient Roman context, from Latin rostrum, the name of the platform stand for public speakers in the Forum in ancient Rome. It was decorated with the beaks of ships taken in the first naval victory of the Roman republic, over Antium, in 338 B.C.E., and the Latin word's older sense is "end of a ship's prow," literally "beak, muzzle, snout," originally "means of gnawing," an instrument noun from rodere "to gnaw" (see rodent).

The beaks were an ancient form of ram, a beam spiked with pointed iron for the purpose of sinking other vessels. For the form, compare claustrum "lock, bar," from claudere "to shut." The extended sense, in reference to any platform for public speaking, is attested by 1766.

The classical plural is rostra, though in English this is more common in the original "ship's beak" sense and -rums often is used in the secondary sense.

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rat (n.)

"a rodent of some of the larger species of the genus Mus," late Old English ræt "rat," a word of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Medieval Latin ratus, Italian ratto, Spanish rata, Old French rat) and Germanic (Old Saxon ratta; Middle Dutch ratte, Dutch rat; German Ratte, dialectal Ratz; Swedish råtta, Danish rotte) languages, but their connection to one another and the ultimate source of the word are unknown. In its range and uncertain origin, it is much like cat.

Perhaps from Vulgar Latin *rattus, but Weekley thinks this is of Germanic origin, "the animal having come from the East with the race-migrations" and the word passing thence to the Romanic languages. American Heritage and Tucker connect Old English ræt to Latin rodere and thus to PIE root *red- "to scrape, scratch, gnaw," source of rodent (q.v.). Klein says there is no such connection and suggests a possible cognate in Greek rhine "file, rasp." Weekley connects the English noun and the Latin verb with a question mark and OED says it is "probable" that the rat word spread from Germanic to Romanic, but takes no position on further etymology. The common Middle English form was ratton, from augmented Old French form raton. Applied to rat-like species on other continents from 1580s.

The distinction between rat and mouse, in the application of the names to animals everywhere parasitic with man, is obvious and familiar. But these are simply larger and smaller species of the same genus, very closely related zoologically, and in the application of the two names to the many other species of the same genus all distinction between them is lost. [Century Dictionary]

Applied since 12c. (in surnames) to persons held to resemble rats or share some characteristic or quality with them. Specific sense of "one who abandons his associates for personal advantage" (1620s) is from the belief that rats leave a ship about to sink or a house about to fall, and this led to the meaning "traitor, informant" (1902).

To smell a rat "to be put on the watch by suspicion as the cat by the scent of a rat; to suspect danger" [Johnson] is from 1540s.  _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864.

RATS. Of these there are the following kinds: a black rat and a grey rat, a py-rat and a cu-rat. ["Dictionary of the Vulgar Tongue," Grose, 1788]  
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jerboa (n.)
small desert rodent, 1660s, Modern Latin, from Arabic jarbu "flesh of the loins," also the name of a small jumping rodent of North Africa. So called for the strong muscles of its hind legs. Compare gerbil.
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