Meaning of “semi-pro”

Question

I was watching The Big Bang Theory series and when Sheldon and Leonard went to a sperm bank and Sheldon changed his mind about giving sperm, Leonard told him

What, are you kidding? You’re a semi-pro.

What does semi-pro mean here?

Answer 1

So, semi-pro is short for semi-professional, and while used metaphorically in this case, doesn't vary greatly from its usual definition of someone who lies between an amateur and a professional. (Strictly speaking, this refers to whether the person is paid to engage in an activity or not, with semi-professionals being paid, but not enough to serve as a full time job. However, in can also simply refer to someone who engages in an activity with a higher degree of skill or higher frequency than an amateur, but less than what would be considered professional)

However, context is critical, and in this case, the exact phrase used by Sheldon was omitted from the question and is key to the joke:

Sheldon: I don't think I can do this.

Leonard: What, are you kidding? You're a semi-pro.

You're correct in understanding that Sheldon means he's changed his mind about going through with it. However, for comedic effect, Leonard interprets it more literally (that Sheldon is saying that he physically is unable to do so), and states that Sheldon is a semi-pro at it (implying that he does it so frequently/expertly as to have attained a higher than amateur status).