Like a guy said all these pretty girls, but I just want to knock off her.

I looked it up and this is what I found:

Knock off
1. Informal
a. To take a break or rest from; stop: knocked off work at noon.
b. To cease work: It's after five; let's knock off.
2. Informal To complete, accomplish, or dispose of hastily or easily; finish: That author knocks off a book a year.
3. Informal To get rid of; eliminate: knocked off 12 pounds in a month.
4. Slang To kill or overcome.
5. Slang To hold up or rob: knocked off a bank.
6. Informal To copy or imitate, especially without permission: knocking off someone else's ideas.

So I guess it could mean he wants to kill her, or get over her, rob her, copy her, take a break from her, get rid of her. Not sure which though.

I would ask him to explain what he means. He used a phrase in a seemingly uncommon way (though it may be very common where he is from) so he would be your best bet to find out.

Other than what Alty said, the only thoughts that come to mind is that our adult language filter blanked a word or he meant 'knock up' which is slang for 'get pregnant'.

In my area "knock off" means "have sex."

For example, "See that girl over there? I'd really like to knock her off."

In my area "knock off" means "have sex."

For example, "See that girl over there? I'd really like to knock her off."
Definitely