A nickname/code word used as an alternative when a girl or woman is on her menstrual cycle or her period.
jane: "hey betty, you wanna come over after school today?"

betty: "nah, i'm having a visit from auntie flo and i don't feel too well about it."
by H.J.S. July 28, 2006
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Auntie flo can also be used in limericks, such as here:

There once was an auntie called flo.
She was sexy and red like Bordeaux.
When she met a vibrator,
She said: come back later,
Cuz today I prefer a regular Joe.
by kinkylimerix January 11, 2015
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