E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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. Prosperines 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 (Poetaster): 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 Poesiè); 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.