give a punch
The German phrase 'eine Kopfnuss geben' literally translates to 'give a head nut', but it is used to mean 'give a punch', specifically to the head. This phrase is often used in a more playful or less violent context, as in 'give someone a playful punch to the head'.
In a fit of rage, he gave him a headbutt.
This phrase describes a situation where someone, in a state of anger, resorts to violence by giving someone else a headbutt, which is what 'eine Kopfnuss geben' translates to in English.
If you're not careful, you'll get a headbutt.
Here, the phrase is used as a warning or threat. The speaker is saying that if the listener doesn't watch their actions, they'll experience the act of 'eine Kopfnuss geben', or receiving a headbutt in English.
The last time I gave him a headbutt, I hurt myself.
In this example, the speaker is expressing regret over a previous incident where they, not someone else, were the ones who ended up getting hurt in the action of 'eine Kopfnuss geben', or giving a headbutt.