A hard knock, a gangster or sumone who leads a hard life or has been through hard times.

Someone who lives a life full of hardships.

Also a song by Jay Z

A term used in the military, for storming a house using violent force. It doesn't nessicerily mean shooting or throwing grenades before entering a room, but the soldiers will be highly alert and ready to do so.
The opposite is a "soft knock" which means knocking on the door and "asking" calmly to come in.

The product of a woman who has had her breasts enhanced via implants. When a woman gets an augmentation of her breasts they normally become hard while at the same time becoming very large. Large breast are often referred to as knockers so hence the name, Hard Knocks, short for Hard Knockers.

Hard Knocks is used simply because it is easier to say than Hard Knockers and woman have no idea what the fuck you mean when you say it.

get hits / smashed / jacked

To learn something (usually a life lesson) the hard way. (Some say the best way to really learn is to learn from your mistakes.)

To have a life that is difficult, where the elements of the world seem to be against you all the time.
For example, life on the streets, the life of a hustler or a dealer.