Idiom phrase with noun

[Verghi]

I saw this expression: "You must be a noun to detect someone's sarcasm" and I'm not sure about the real meaning of noun in this context; I thought it means something like "anyone could detect someone's sarcasm", but I can't find a support for my hypothesis anywhere. Could you please clear my mind about it? Thanks.

[Mordant]

without knowing the whole context it would be difficult but sometime we say

You must be a nun if you didn't detect that sarcasm, or didn't get the joke, or didn't see the evil look. Assuming the other person naive.