Literally "there's a worm in it." This phrase describes a situation where things keep going wrong or nothing seems to work properly. It refers to something being fundamentally flawed.
The expression originates from fishing, where having a worm in your bait box that isn't lively enough won't attract fish, leading to unsuccessful fishing. Another theory ties it to woodworking, where worm-infested wood is weak and problematic. The phrase has been in common use since at least the 19th century and remains one of Germany's most frequently used idioms.
🇩🇪 – Heute ist der Wurm drin. Erst verschlafe ich, dann fällt der Zug aus, und jetzt regnet es auch noch. 🇬🇧 – Things are going wrong today. First I oversleep, then the train gets cancelled, and now it's raining too.