裤衩终究挡不了大姨妈
[kù chà zhōng jiū dăng bù le dà yí mā]
Ultimately Underwear Can't Stop Aunt Flow. 'Aunt Flow' refers humorously to menstruation in English, while the whole saying uses underwear to metaphorically represent a weak obstacle against something powerful and inevitable like nature's course, especially highlighting the menstrual cycle.