means A mild exclamation of annoyance or disappointment, equivalent to 'darn' or 'drat'; also the plural of rat, the long-tailed rodent.

from As the rodent, 'rat' descends from Old English 'rætt,' with relatives across the Germanic languages and probably beyond — the word, like the animal, turns up nearly everywhere, though its deepest root is murky. The exclamation 'rats!' is a later, lighter spin: an American slang minced oath from around the late 19th century, calling on the despised vermin as a tame, printable stand-in for stronger curses — much like 'rats' on something or someone meant nonsense or contempt.