I suspect that this comment has nothing to do with fish and chips at all. My guess is that it's merely a colourful expression to indicate surprise. If the person saying it has just been told some very surprising information, he or she says this to mean 'Well! That's amazing. I'd never have believed that!' It follows the same pattern as the slightly old-fashioned expression 'Well, blow me down!', also used to indicate surprise. My other guess is that it's set in Scotland. The term 'fish supper' is mainly used there. If you can tell us what film it's from and what the previous person has said, we can probably help you a bit more.

It's a line from the movie "What we did on our Holiday" spoken by Billy Connolly and set in Scotland. It just means "I'm surprised at that" or "Thats Amazing". Basically an expression of surprise.

It is a saying that expresses surprise at something. For example, you might meet a long lost friend on the train, someone who you believed had moved abroad and you thought you would never see again. When you meet you might say this phrase as a way to express how surprise and pleased you are. There are many different phrases like this, sometimes people just make them up, although they need to be absurd in order to work.

Without the context it's hard to guess (which movie was it?) but sometimes people say that sort of thing as an expression of surprise. I've never heard anyone say those exact words, though.