The word "大姨妈" is formed by combining the characters 大 (big), 姨 (aunt), and 妈 (mother), where "姨妈" specifically denotes a maternal aunt. This term serves as a euphemism for menstruation, derived from a cultural metaphor where women would discreetly indicate their period by saying "大姨妈来了" ("Big Aunt has come"), thus personifying the condition to circumvent direct discussion due to social taboos.