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.
"damn that girl has fake jacks for sure"
"ya, those are definitely hard knocks"