在装逼你就是姐的大姨妈
[zài zhuāng bī nĭ jiù shì jiĕ de dà yí mā]
This humorous phrase is informal and slang-heavy, implying 'when acting arrogantly, then you become my period.' It is used sarcastically to dismiss or belittle someone showing off excessively or being obnoxiously cocky.