“Smoking is definitely a deal-breaker.”

You're talking with your friend about what kind of guys you would like to date. You would never date someone who was a smoker, so you say:

Smoking is definitely a deal-breaker.

definitely

Use "definitely" to describe something that you're really sure about.

a deal breaker

A "deal breaker" is a quality or point that you can't accept. Whenever you're considering a decision, there are some things you don't like but can live with. But a "deal breaker" is something that you can't live with. You use this phrase to talk about negotiations in business situations:

We liked their work, but the rates they were asking for were a deal-breaker for us.

This sentence means that you definitely won't use this company's services, because they're too expensive.

Or you can use "a deal-breaker" to talk about relationships like in the example at top:

Smoking is definitely a deal-breaker.

In that case, it means that you definitely won't date someone who's a smoker.