Christina Milian Is Gonna Knock You Out with REAL Ring Skills

Don't call it a comeback, but Christina Milian's quick hands are back in a boxing ring ... and it's more than just a great workout, because the sweet science is in her blood.

We're told she started taking classes this week with her trainer, Jason Ross, at LA Fitness in Hollywood. It's the first time she's laced up the gloves in years, but her grandfather and father both boxed and taught her from childhood.

The video's pretty sick -- Christina's already throwing strong combinations. We're told it's mostly about fitness now, but she's not ruling out sparring eventually ... with very thick headgear, we hope.

Don't mess with yer moneymaker, Tina!