Clunk (n): The act of killing in one or finishing off an opponent in halo with a strike to the back of the head/back of the face/right in the spine with the melee attack of your chosen weapon. It is best used having approached your opponent unawares, in a sniping match for example and having a "clunk" as the final kill is particularly gratifying.
Also as verb: "to Clunk" and as an exclamation having completed the above action. See also wingo
As noun: "That was such a clunk"
As verb: "I clunked you soooo hard!"
As exclamation following the clunk: "Clunk!" (often pronounced "cloonk"!)