The Egyptians and Phrygians deified rats. The people of Bassora and Cambay to the present time forbid their destruction. In Egypt the rat symbolised “utter destruction;” it also symbolised “judgment,” because rats always choose the best bread for their repast.
Rat. Pliny tells us (bk. viii. ch. lvii.) that the Romans drew presages from these animals, and to see a white rat foreboded good fortune. The bucklers at Lanuvium being gnawed by rats presaged ill-fortune, and the battle of the Marses, fought soon after, confirmed this superstition. Prosperine's veil was embroidered with rats.
Irish rats rhymed to death. It was once a prevalent opinion that rats in pasturages could be extirpated by anathematising them in rhyming verse or by metrical charms. This notion is frequently alluded to by ancient authors. Thus, Ben Jonson says: “Rhyme them to death, as they do Irish rats” (Poctaster): Sir Philip Sidney says: “Though I will not wish unto you ... to be rimed to death, as is said to be done in Ireland” (Defence of Poesie); and Shakespeare makes Rosalind say: “I was never so berhymed since ... I was an Irish rat,” alluding to the Pythagorean doctrine of the transmigration of souls (As You Like It, iii. 2). (See Charm.)
I smell a rat. I perceive there is something concealed which is mischievous. The allusion is to a cat smelling a rat.
(To). To forsake a losing side for the stronger party. It is said that rats forsake ships not weatherproof. A rat is one who rats or deserts his party. Hence workmen who work during a strike are called “rats.”
Averting ... The cup of sorrow from their lips. And fly like rats from sinking ships.
Swift: Epistle to Mr. Nugent.
(Un). A purse. Hence, a young boy thief is called a Raton. A sort of pun on the word rapt from the Latin rapto, to carry off forcibly. Courir le rat, to rob or break into a house at night-time.
To take a rat by the tail, or Prendre un rat par la queue, is to cut a purse. A phrase dating back to the age of Louis XIII., and inserted in Cotgrave's Dictionary. Of course, a cutpurse would cut the purse at the string or else he would spill the contents.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894