It meant a punishment or a response to match the original affront. Supporters of capital punishment have used it to justify killing murderers.
Presumably they are not Christians because, in the Sermon on the Mount, Jesus rejects this and says that, on the contrary, if someone smites you on the cheek you should turn the other one to be smitten also.
Which makes one wonder why Jesus is considered a wise man. Just imagine it...
"Oh, you've stolen a tenner from me? Hang on, there's another tenner in the other pocket."
"You've raped my daughter? Well, I've another daughter here. Would you like to rape her?"
Isn't religion wonderful?