In Chapter 8, Inspector Japp uses an idiom while talking to Alfred Inglethorp, seemingly commiserating with him about the rumors surrounding his status as a murder suspect:
“Between you and me, sir,” remarked Japp, “I’d sooner have any amount of rumours than be arrested for murder. And I venture to think that your poor lady would have felt the same. And, if it hadn’t been for Mr. Poirot here, arrested you would have been, as sure as eggs is eggs!”
The idiom in the passage above—"as sure as eggs is eggs"— is a short phrase that conveys utter certainty. Inspector Japp’s employment of this idiom reveals the surety that he and the other detectives had in Inglethorp's guilt prior to Poirot’s intervention preventing his arrest, even as he chats somewhat amiably with the man. The inspector’s quick shift in thought towards the novel’s prime (and true!) suspect reveals his own gullibility and the flaws in the local detectives’ training when it comes to solving crime. The particular language of this idiom is quite colloquial, further reflecting his status as a bumbling detective who is out of his depth at discovering mysteries. Japp’s reliance on Poirot’s expertise and his deference to Poirot’s judgment emphasizes just how remarkable the Belgian detective is in comparison to his professional colleagues.
During the final chapter, as The Mysterious Affair at Styles comes to a close, Poirot uses an idiom to explain to Hastings why he kept him in the dark regarding his suspicions of the murderer's identity:
If I had told you my ideas, the very first time you saw Mr. Alfred Inglethorp that astute gentleman would have—in your so expressive idiom—‘smelt a rat!’ And then, bonjour to our chances of catching him!
In the passage above, the idiom “to smell a rat” is used to indicate that one suspects trickery, deception, or foul play. Poriot's assertion that letting Hastings in on his suspicions regarding Alfred Inglethorp would immediately result in Mr. Inglethorp’s own subsequent suspicion of Hastings is significant. It is extremely telling that Poirot was able to ascertain Hastings’s lack of skill in deception so early on in the novel. Poirot’s lack of confidence in Hastings’s abilities reflects his view of Hastings and his bumbling, guileless nature, demonstrating the fact that he has never been cut out for true detective work. With this short, simple idiom, Poirot accurately and sharply critiques Hastings’s investigative flaws (which would have immediately revealed his suspicions and therefore tanked the investigation), while still remaining lighthearted enough that he refrains from resorting to outright insults.
