Green’s Dictionary of Slang

(US) menstruation.

1954		‘Miscellany’ AS XXIX:4 298: Reference to Visitors or Person [...] Aunt Flo is come.
1980		Maledicta IV:2 (Winter) 229: Flo suggests sodomy for men and menstruation (falling off the roof and bleeding) for women.
1989		P. Munro Sl. U.
1999		J. Randall ‘A Visit from Aunt Rose’ in Verbatim XXV:1 Winter 25: Generally the period takes on the identity of a friend or relative, usually female, who comes for a visit: [...] Aunt Flo (i.e., flow, incorporating the idea of the flow of blood).
2007		C. Palahniuk Rant 21: Those gusty days, it’s everybody’s Aunt Flo has come to visit. Pads and napkins walking off.
2014		N.Y. Times 24 Apr. 🌐 Our name [i.e. HelloFlo tampons], while it’s bold, speaks to a very specific audience: women who say, ‘My Aunt Flo is in town’.
2015		mumsnet.com 🌐 Acornyms List [...] AF Auntie Flo (old-fashioned schoolgirl slang for period).
2017		T. Spicer Good Girl Stripped Bare 92: Aunty Flo has missed her monthly appointment. ‘We’re so disappointed she hasn’t arrived. We were expecting her to bring hot scones, with jam and cream, along with the lining of my uterus’.
2024		Bluesky 16 Mar. 🌐 Aaahhhhhh officially menopausal. March 15 marks one year without a visit from Aunt Flo. Fuck you, you old bag! See you never!