Where did the term knocked up come from? I have done some searching, but alas, no luck.
Seems straight forward enough. You KNOCK on someone’s bedroom door to wake them UP. Sort of like WAKING someone up, only by knocking. I’ve only ever heard Brits use it, as in, “Brilliant! You sleep it off here tonight, and I’ll stop by your house on my way to Cheshire and knock your wife up so she can meet you at church tomorrow.”
Glad to be of service.
**Inigo’s ** post, which I am sure is tongue in cheek, illustrates the dangers of different slang usages in the UK and the US. Similarly, when I was living in New Zealand, I was quite startled when the office secretary, a respectable middle-aged woman, asked me if I needed some rubbers. 
Dare you suggest I would post jape as the initial response to a virginal GQ? You wound me!
It hurts me in my heart!
I think I’ll cry.
No kidding, and the same goes for Australia. I was pretty surprised one day to hear someone mention that their border collie was knocked up (tired out) after a hard day’s work herding sheep.
On a related note, a friend of mine was startled when a colleague’s wife said to him, “Oh, you’ve come straight from the airport after that long flight? You must be buggered.” 
According to the OED, “knock” meant “to have sex with” as far back at 1598. It later meant “to become pregnant” from the phrase “to knock a child out” or “to knock an apple out.”
“Knock up” didn’t mean “make pregnant” until 1813 or so.
Maybe it’s just a regional variation, but until I was an adult I never heard any other term but rubbers for the things you put on over your shoes when it’s raining.
Now it has three meanings. I’d forgotten about the shoe-kind, even though they’re really handy.
Yes, when I was a kid that was the most usual use of the term. But after becoming an adult I hadn’t heard it used much except for the kind of rubber you wear somewhere else.
But what my friend in NZ was referring to was neither of these, but erasers.
Funny. I just got that after Speaker for the Dead talked about three meanings. And I know the British usage, too.
Oh well. Can I just say that I’m sick and was up half the night?
Hmmm, I can now think of five meanings for ‘rubber’.
Playing Bridge?
Same here.
In Australia too.
Every morning in primary school Sister would say “now children, get out your pencils and rubbers.”