decapitate
刎 centers on cutting off a head: decapitation. The meaning is specific and violent, with no broad abstract extensions.
刎 combines 勿 (a pictograph of a knife or cutting tool) with 刂 (knife), reinforcing the idea of cutting or severing. The specific meaning of decapitation likely developed from this cutting sense.
The left side 勿 looks like a blade, and the right side 刂 is a knife. Together they form a double-bladed cutting action strong enough to decapitate.
For フン, imagine the sound of a swift decapitation: a sharp 'fun' sound as the blade cuts through. Fun -> フン, and the head falls.