also musk-rat, "large aquatic rodent of North America," 1610s, alteration (by association with musk and rat) of an Algonquian word (probably Powhatan), muscascus, literally "it is red," so called for its coloring. From cognate Abenaki moskwas comes variant form musquash (1620s). Dialectal mushrat is by 1890.
1550, "the mental process of reasoning," later, "the relation of one number to another" (1660s), then "fixed allowance of food or other means of subsistence for a fixed period of time" (1702, often rations, from French ration in this sense).
All are from Latin rationem (nominative ratio) "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," in Medieval Latin "a computed share or allowance of food." This is from rat-, past participle stem of reri "to reckon, calculate," also "think" (from PIE root *re- "to reason, count").
The first sense listed for the English word is obsolete; the second has gone with ratio. The military pronunciation (rhymes with fashion) took over in English from the preferred civilian pronunciation (rhymes with nation) during World War I. That war also gave the word a specific sense of "officially limited allowance for civilians in times of war or dearth" (by 1917).
c. 1600, from German Hamster, from Middle High German hamastra "hamster," probably from Old Church Slavonic chomestoru "hamster" (the animal is native to southeastern Europe), which is perhaps a blend of Russian chomiak "hamster," and Lithuanian staras "ground squirrel." The older English name for it was German rat.
late Old English hwearf "shore, bank where ships can tie up," earlier "dam, embankment," from Proto-Germanic *hwarfaz (source also of Middle Low German werf "mole, dam, wharf," German Werft "shipyard, dockyard"); related to Old English hwearfian "to turn," perhaps in a sense implying "busy activity," from PIE root *kwerp- "to turn, revolve" (source also of Old Norse hverfa "to turn round," German werben "to enlist, solicit, court, woo," Gothic hvairban "to wander," Greek karpos "wrist," Sanskrit surpam "winnowing fan"). Wharf rat is from 1812 as "type of rat common on ships and docks;" extended sense "person who hangs around docks" is recorded from 1836.
by 1953, name for human anti-coagulant use of the rat poison warfarin sodium, abstracted from the chemical name, 3-(α-acetonylbenzyl)-4-hydroxycoumarin; earlier known as Dicoumarol, it attained publicity when it was used in 1955 to treat U.S. President Dwight Eisenhower after a heart attack. Coumarin as the name of an aromatic crystalline substance is by 1830 in English, from French coumarine, from coumarou, the native name in Guyana of the tonka or tonquin bean, one source of the substance.