小三好比大姨妈
[xiăo sān hăo bĭ dà yí mā]
This nickname uses colloquial Chinese expressions to criticize an affair or extramarital relationship, equating '小三' (the mistress) to '大姨妈' (a euphemism for menstrual period, often perceived negatively in this context). It conveys a disdainful attitude towards the situation.