He had the courage to ask her out for a date.
For some reason, using the Spanish word for them makes it less vulgar so you'll hear that in the US as well.
By considering those male bits as the source of a man's courage and strength, if a man has the courage and strength to do something, you say he had the balls to do it.
Not an ESL teacher
I think it is also important to state clearly that this expression, although largely used with the meanings already explained, is highly biased and implies the prejudice that men are more courageous and strong (in several senses) than women.
Prejudice? ;-)
The OP's sentence seems to refute this. (Seems to me, but that could be caused by my prejudice that most people are heterosexual.)Finally, for the learners, it should be clear that, in normal conditions, this expression is used solely by men.
Maybe you meant "used to describe men"? Well, I think that's not entirely true either (see this for example).
Sorry fivejedjon. I don't know if I used the wrong word or if you are just joking and being sarcastic.
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Someone dares to do something or has courage to do something...