The Brave Little Slot Machine

I was looking over a story students were writing in a group. I saw the following sentence (in French): "The slot machine only had one arm, nevertheless it was very courageous."

Me: Why are you talking about a slot machine being courageous?

Students: It’s supposed to say “one-armed bandit”. We found it in the dictionary.

Me: A “one-armed bandit” is a slang expression for a slot machine. Why on earth would the dictionary give you an expression for “one-armed bandit”?

Students: [No response]