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.)).
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.