'Big fat bitch nugget' means if someone is fat, rude or gross.
Bob: Karen your a big fat bitch nugget!
Karen: Oh my god don't say that I am no big fat bitch nugget you Bitch nugget!
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... means that something is going a bit downhill or just awkward.
Bazza: OH MY GOD I LOVE CHICKEN NUGGETS AND PEANUT SAUCE!
Bob: ...
Bob said ... because he is confused.