How do you say Holy cow in German?

There isn't a single perfect equivalent of "Holy cow!" in German that captures the same casual, surprised, and slightly irreverent tone. However, depending on the context and the level of surprise/excitement, you could use several options:

* Mein Gott! (My God!) - This is a common and fairly direct translation, but it's more religious than "Holy cow!"

* Himmelherrgott! (Heaven's God!) - Similar to "Mein Gott!" but slightly more emphatic.

* Du meine Güte! (My goodness!) - A more gentle and less religiously charged option.

* Unglaublich! (Unbelievable!) - This focuses on the surprising aspect.

* Wahnsinn! (Madness!) - This emphasizes the craziness or unexpectedness of the situation.

The best choice depends on the specific situation. For a general equivalent with a similar casual feel, Du meine Güte! or Unglaublich! are probably the closest.