Telling your son not to “be such a girl” lets his sister who overhears the conversation know that being a girl is not a good thing and she should be sorry and ashamed of herself.
It also reminds your son that being a boy is better than being a girl and therefore he is better than any girl he will ever meet.
In russian if you cry you get called a cabbage. Vote to change “dont be such a girl to “dont be such a cabbage” say I.
I wish that there was a socially acceptable way to say, “I’m having a bad mental health day and need you to pay attention to me,” without alienating everyone.
or: “I’m having a bad mental health day and need to be on my own for a while so please don’t be mad if I cancel our plans on short notice.”