Why can two be used as a curse?

Probably derived from the dialect described in Baidu Encyclopedia:

Shaanxi Dialect

1. Often used to describe a person who cannot distinguish between occasions and cannot decide what to do. No matter how serious it is, you can’t speak without thinking, and you can’t do things without thinking about the consequences. Such as "two cowards, two hammers". Example: That person is very good! 2. Describe your psychological state when you hear shocking and incomprehensible remarks, such as "I'm confused." 3. Describe a certain aspect as relatively strong, with a sense of humor. Example: That guy is very good at football.

Northeast dialect

It is often used to describe people or things as stupid, and has a unique semantic meaning in spoken language. Often with a derogatory connotation.

Baoding Dialect

Derogatory term. Often used to describe people who are stupid or mentally retarded. Nowadays, it is mostly used to make fun of each other (with a lot of joking content). Example: 1. In math class, the teacher asked: "Have you done all the questions? What is the answer?" Classmate: "Two!" Teacher: "I think you are only 'two'! You can even calculate this question wrong, what do you think?" How many "twos" do you have!" 2. The extended meaning of two hundred and five means, I don't know, don't understand, don't understand, don't know, don't know.

Jiangxi dialect

It is the Nankang Hakka dialect in Ganzhou area. 2. "Er Zai" and "Er Thief" are nicknames for children and are also used as greetings between friends. Meaning: Fool, idiot, stupid