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.
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.
Jay-Z's "Hard Knock Life"
It's a hard knock life for us, it's a hard knock life for us, instead of treated, we get tricked, instead of kisses, we get kicked, it's a hard knock life