Kuhli platt machen uses everyday German slang rather than a formal phrase. Platt machen can mean to flatten something, beat someone clearly, or shut an argument down, depending on context. Kuhli appears to be a name or nickname in this clip, which makes the line feel personal and oddly specific. That specificity helped it circulate as a soundboard clip, where short foreign language threats are often reused out of context for comic effect. It is still used because the delivery sounds serious while the situation it is pasted into is usually trivial.