姨妈 ('auntie, maternal aunt') is the all-purpose slang euphemism for the menstrual period - 'my auntie's visiting' (大姨妈来了). 姨妈巾 therefore = period pad. Very common in everyday speech; 卫生巾 is the neutral/formal term used on packaging.
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Colloquial; fine in chat and conversation. Formal writing (medical, retail, product copy) uses 卫生巾.
