Ich verstehe nur Bahnhof

Literally: I only understand train station

Example: “Der Professor sprach zwanzig Minuten über Statistik, und ich verstand nur Bahnhof.” — The professor talked about statistics for twenty minutes, and it was all Greek to me.

English equivalent: It's all Greek to me

Origin: The explanation given by the Duden dictionary of idioms, which presents it as likely rather than certain, goes back to the end of the First World War. Worn out by hopeless trench warfare, German soldiers wanted only to go home, and the railway station (Bahnhof) was where the journey home began. So set were they on it that whatever else was said to them, they "understood only station": any talk on another subject was cut off with ich verstehe nur Bahnhof. Other explanations exist, but this is the standard one.