Whether it's a canary in the coal mine or a waitress in the weeds, idiomatic expressions can sometimes stump us even in our own language. What common expression puzzles you the most?
"Off the wagon" / "On the wagon"
What do wagons have to do with alcoholism? Well, I didn't know the origin of that expression until I did a web search just now. There's also an episode of Seinfeld about the phrase.