expressing incredulity, disappointment, annoyance, etc., 1886, American English, from rat (n.).
"resembling a mouse or rat," c. 1600, from Latin murinus "of a mouse," from mus "mouse" (see mouse (n.)).
"a fin-footed mammal," one having feet like fins or flippers, especially of the group of fin-footed aquatic carnivorous quadruped mammals that includes seals, sea-lions, and walruses, 1842, from Modern Latin Pinnipedia, suborder of aquatic carnivorous mammals (seals and walruses), literally "having feet as fins," from Latin pinna in its secondary sense "fin" (see pin (n.)) + pes, genitive pedis "foot" (from PIE root *ped- "foot").
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.