MEANING

to smell a rat: to detect something suspicious

ORIGIN

The first known use of this phrase is in The Image of Ipocrysy, an anonymous poem written around 1540, denouncing “the cruell clergy”:

(published in 1843)
Suche be owr [= our] primates,
Our bisshopps and prelates,
Our parsons and curates,
With other like estates
That were [= wear] shaven pates;
As monkes white and blacke,
And channons that cane chatte,
Glottons ffayre and fatt,
With ffriers of the sacke,
And brothers of the bagg,
As nymble as a nagg,
That cane bothe prate and bragg,
To make the pulpett wagge
With twenty thousand lyes,
Do make the blind eate flyes,
And blere [= blear] our symple eyes,
To make vs to beleve
God morowe is god eve;
For pleynly to be breve [= brief],
So nye [= nigh] they do vs dreve [= do disturb us],
That we, to our great greve [= grief],
Must sey that white is blacke,
Or elles [= else] they sey we smacke,
And smell we wote [= know] not what:
But then beware the catt;
For yf they smell a ratt,
They grisely [= grimly] chide and chatt,
And, Haue him by the jack [= jacket],
A fagott for his backe,
Or, Take him to the racke,
And drowne hyme in a sacke,
Or burne hyme on a stake!

The presence of “the catt”, a metaphor for the clergy, immediately before the phrase suggests that to smell a rat originally referred to a cat knowing a rat is present from its smell without being able to see it, and that this idea was transferred to a person who had cause to feel that something was not as it should be.

This seems to be confirmed by the following passage from the second part of The honour of the gentle craft, written around 1598 by the English author Thomas Deloney (circa 1543-1600). Robin has just told Meg that his master, Richard Castler, desires to meet her in Tutle fields and that she must make sure Gillian is not there. Robin has also separately told Gillian that Richard wants to meet her there and that Meg must be absent. The two maids therefore

were picking hearbs in the fields: when Meg saw that Gillian would not away, she came to her, and askt her what she did there? nay what do you here quoth she? quoth Meg I come to gather thrift, but I can find nothing but thistles: quoth Gillian but I mean to get hearts-ease ere I goe: Nay quoth Meg I shall find thrift, as soon as you shall find heart-ease, but I think I shall find none to day, then get you home quoth Gillian, would you be rid of my company qd. Meg? for that word I mean not to be gon yet: I faith Gillian I smell a Rat. Then quoth she you have as good a nose as our gray Cat: but what Rat do you smell tell me? I doubt, if there be any Rat in the field, you would fain catch him in your trap, but I faith Meg you shall be deceiv’d: then qd. Meg you would have the Rat taste no chéese but your own; then said Gillian, wheresoever he run, I would have him créep into no corner of yours: you speak mysticall quoth Meg: if thou art a good Wench let’s go home together: quoth Gillian, as I came not with you, so I mean not to go with you. (1660 edition)