A corker was something which was a bit of a showstopper - e.g. something which applies an air of finality to a discussion or activity. Hence the term "corker" - to put a stopper in something. The meaning has chnaged in later years to apply to any remarkable act or example.
I can also mean a lie - an especially good one at that - rather like a whopper.
The two applications of the word may have two origins, the former, as suggester from the action of corking something, the latter from the Romany word "khokhav" meaning to deceive.