What does the slang word darn mean?
It is a euphemism for the word damn.
The slang term 'darn' is usually used to let out a signal of disappointment. For example, if someone was told that their favorite football team had lost a game in the last minute, an appropriate response of the person could be "darn" as they would be sad their team lost. The term is also used as a substitute for the word "damn".