Here's the description from the "Stupid Questions" site:
The use of “frog” against French people is generally dated to a quote in Fanny Burney’s 1778 novel “Evelina”: “Hark you, Mrs. Frog…you may lie in the mud till some of your Monsieurs come to help you out of it.”
Some sources attribute this to the Parisian coat of arms, which depicts three toads. But it’s now accepted that it comes from the French habit of eating frogs, which the British consider strange. This theory finds support in the related slur “frog-eater,” dating to the early 1800s.
“Frog” entered US slang during World War I.
And Poirot entered the United Kingdom just after that war....