Where does ‘knocked up’ come from?

Anthony Horowitz (Diary, 4 February) tells us he was advised by a ‘sensitivity reader’ to remove the word scalpel from a book with a Native American character lest it suggest scalps (though the words are unrelated).

I’ve stumbled across the birth of a new forbidden phrase on Twitter, that social media swamp for the older swampster: knocked up. A California lawyer called Johnathan Perk declares in a tweet: ‘The phrase “knocked up”, referring to pregnancy, originated with US slavery. The Oxford English Dictionary traces the expression back to 1813. Back then the price of enslaved African women was “knocked up” by the auctioneer when she was pregnant – promoted as a deal for buyers.’ Even the evidence he produces shows this is not the case, but his tweet has been ‘liked’ 28,000 times. Now hundreds of people believe this story and many will think those who deny it are ‘slavery apologists’.