You lot = the whole lot of you?
Doxy = a strange creature like a hornet?
From Harry Potter:
"Right, you lot, you need to be careful, because Doxys bite and their teeth are poisonous."
Yes, you lot = all of you.
A doxy used to be a prostitute, but I don't suppose Harry has those in mind-- perhaps as he grows older in the next book of the series. I would be careful of trying to use this word, NH-- it is familiar only to HP fans, and will likely die a quick death as the series palls on the public.
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