What it is:
D-mark is slang for deutschmark, the national currency of Germany until it joined the European Union in 2002.
How it works (Example):
For example, let's say you're at a cocktail party looking to impress some Wall Street currency traders. If you want to sound like you know what you're talking about, you could say something like, "I bet Chancellor Merkel is bummed that she didn't stick to the D-mark. The rest of the EU is really dragging down the euro."