大姨媽

Etymology
The euphemistic sense was originally a colloquialism.

Noun

 * 1)  eldest maternal aunt mother's eldest sister older than one's mother
 * 2)  Aunt Flo a personification of the menstrual period or menstruation

Usage notes
If one's mother is the eldest of the sisters, her oldest sister is not reckoned as a 大姨媽 but as 二姨.