A fanny nanny is someone who carries around a fanny pack. The "nanny" part refers to when for instance a group goes on a rollercoaster and needs a place to put their phones they ask the friend with the fanny pack to carry their phones.
Elena: Hey, Victoria could I put my phone in your fanny pack during the ride?
Adrian: Yeah me too!
Lacey: Me as well!
Sarah: if you don't mind...
Victoria: *sigh* I guess I'm the fanny nanny today.
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