Today's weird history. There once was an profession called knocker. These men went around the village in the morning and knocked on people's bedroom windows to wake them up for work. Before alarm clocks. They were paid for this. If you needed their service on a particular day, you'd tell them to knock you up in the morning. The phrase still is part of the language in England, which can be embarrassing for a British lady visiting America, where being knocked up has a totally different meaning.
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