"Ánfora" has a broad meaning, but the word "anforita" is used especifically for a hipflask, because it's small enough to fit in the back pocket of pants or in the inner pocket of a jacket.
I don't know where "nalguera" is used, but I guess it's the back pocket what would give it that name. For what I see in the DRAE, that might be the same origin for "petaca", since that's a slang word for the buttocks too, but I definitely have never heard that word used for a hipflask in Mexico.
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