There are a couple (at least) possible meanings depending on the context. Theatrical people will use this phrase to wish an actor well because they believe it is superstitious to wish someone 'good luck'. By wishing them the opposite (a broken leg), they superstitiously expect the opposite to occur (good luck). So, in this context, it would be appropriate to use the phrase. But this phrase can also be used to encourage a person to 'get moving' or 'get going' to encourage them to take some desired action. For example, when a child is taking a long time to get ready for school, a parent might say to them 'break a leg' to encourage them to move more quickly. In this context, the phrase might be negative, implying a lazy attitude.

It’s origin is in the theater. You very occasionally hear it otherwise, but I’d only use it in connected to acting.

It is used by actors as you'll see when you google it. So if you're an actor and encouraging your colleague as they go on stage then yes. But otherwise it'll sound odd.