中英語のrat, mouseから、古英語のraet, ratから、原語ゲルマン語*ratto, to scrape, to biteから、PIE*red, to scrape, to biteから、語源的にはrodent, eraserから。
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]Middle English common form was ratton, from augmented Old French form raton. Sense of "one who abandons his associates" (1620s) is from belief that rats leave a ship about to sink or a house about to fall and led to meaning "traitor, informant" (1902; verb 1910). Interjection rats is American English, 1886. To smell a rat is 1540s; "to be put on the watch by suspicion as the cat by the scent of a rat; to suspect danger" [Johnson]. _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864.