How to answer "chicken or fish"?
When one person put this question "chicken or fish? " to another person., how to anwer it, I mean, when to say chicken and when to say fish? Which one is positive?
And where did this often-used question come from?

Thank you for answering.To put more details, Gabby sent her ex-husband an invitation for her wedding, but received no reply, so she came to meet him and asked him whether he could make it or not, so "chicken or fish"? ( in Desperate Housewives)

Question

How to answer "chicken or fish"?
When one person put  this question "chicken or fish? " to another person., how to anwer it, I mean, when to say chicken and when to say fish? Which one is positive?
And where did this often-used question come from?

Thank you for answering.To put more details, Gabby sent her ex-husband an invitation for her wedding, but received no reply, so she came to meet him and asked him whether he could make it or not, so "chicken or fish"? ( in Desperate Housewives)

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When she met her Ex-husband to ask if he was coming or not. she didn;t directly ask him. Asking asking him "chicken or fish?", was her round about way of asking if he is attending or not.  

The meal at the wedding reception had Chicken and fish being served. In his answer, he was to choose one or say he was not attending.

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Thank you all, I think fakeivy's answer is perfect. It helps  to understand the metaphor.

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I agree with the others that it's probably a context thing, but if it's meant metaphorically, then maybe chicken means to be a coward. Some fish swim against the currents i.e. face adversity, so it would mean to be brave. In that sense, "Chicken or fish?" would mean "Coming or not?" or "Are you a coward or are you brave?"

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I am following this because I have no idea of any special meaning.
But seeing where it came from... I suppose that there is some special meaning which has to do something with that episode or the series in general. A context thing...

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I don't know the context, but I do not think I have ever heard anyone use this question beyond its literal meaning.