Something, most likely a conversation or an interest in something, that is killed instantly with a single stupid or insanly dull comment. Instead of saying "The conversation has been killed dead", you would say "The conversation has been Ratman'd".
Group of friends - Hey, how does everyone fancy going to the Youth Club?
Ratman (In incredibly whiney and 'desperate for attention' tone) - I once went to a youth club...
(Dead Silence in the confusion of what has just transpired)