One of my favourites has always been "mon chou" (my cabbage) as a term of endearment in French. True, "chou" can also refer to a pastry, and therefore something sweet and delicious, but I prefer the vegetable explanation. In a similar vein, it's tough to explain why "être dans les choux" (to be in the cabbage) means feeling distant or being in trouble. According to some , the reason is that "chou" sounds like "échouer" (to fail).
Another expression I encountered comparatively later in life was "avoir du chien" (to have some dog). Again a counterintuitive concept, it means to have style, or to be sexy. When I first heard someone use it, however, I thought it meant something like being assertive and energetic. There are different suppositions as to how dogs came to symbolize sexiness. Some report that "dog" was a quality attributed to talented actors in the theatre. Others believe that the association comes from the mischievousness and playfulness of the dog.
One intriguing idiom from Québec is "le bonhomme sept heures" (the 7 o'clock man), a frightening character parents once invoked to get their children to exhibit desired behaviour. As a young girl, I thought this "bonhomme" always had to show up at 7, but in fact the expression as a whole is a contraction of the English term "bone setter". Itinerant doctors would show up to set the broken limbs of patients in various locations. Patients' resultant screams would invariably frighten children, so parents took advantage.
English endearments can be just as strange as their French counterparts: why would anyone be flattered by being called a pumpkin? A "sweet nothing" in itself is equally odd, when you think of it.
Meanwhile, I don't mean to "let the cat out of the bag", but the bag in question was supposed to contain a pig obtained in a medieval market place. Dealers would substitute a cat for the pig on display and the shady switcheroo would only be discovered on arrival.
All in all, idioms enrich language even as they often stump the efforts of translators. They certainly lead to entertaining discussions, in any language of choice.