What does "aunt flo" mean?

A personification of the menstrual period or menstruation.

What is a synonym for "aunt flo"?

Common synonyms include: menstruation.

/ɑːnt fləʊ/ name, slang

name, slang ·Uncommon ·College level

Proper Noun

1
A personification of the menstrual period or menstruation. euphemistic, slang
"Aunt Flo came to visit last night: that's why I was so ratty."

"Aunt Flo came to visit last night: that's why I was so ratty."

Pun on flow and Flo (nickname for Florence, Flora, etc).