It's a term used when your party is going to go through its last experience points chain. So killing will continue until you kill a mob and the chain resets. After that, the party usually ends.
'Last chain' is a phrase used when a particular member, usually a leader states a disbanding of the party after next experience chain. By 'experience chain' we understand six mobs killed in a rapid succession, therefore gaining a bonus experience. --Awangarda 14:50, 25 May 2009 (UTC)