It's one of those expressions which can raise a smile, or get you in to an awkward embarrassing moment if used in the wrong place. In the UK it simply means to put or get something together quickly - 'I'll knock you up some breakfast'. It can also mean to wake someone up or to knock on their door. 'I'll knock you up on my way to work' - 'I'll call on you on my way to work' Say this in the US or Canada, though, and you will get an amused or perplexed look, as it means 'to have sex with/to make pregnant' - you can see the possibilities for misunderstanding here !

If a lady is "knocked up", she is pregnant.

I read this meaning of knock up in urbandictionary:wake up ( Great Britain ) "Billy go and knock up your sleppy head brother."

Yes, in America it is slang and not usually used in a positive way (the person probably isn't married and you don't think very highly of the fact that they are pregnant)

It has two meanings in England that I know of 1) to become pregnant or to make someone pregnant 2) to visit someone's house