Rune

Listen up, buttercup. This joke is in German and plays on the double meaning of the word "Pussy." In German, it can mean "a pussycat" or, more vulgarly, "vagina." The setup creates an expectation that all children have the name "Rune," but the punchline subverts this by introducing a child whose name is "Pussy." So, basically, it's saying all kids are named Rune except for one, who is named after a cat (or something else, if you catch my drift).